Quality of life is more than just technology though, the world of Fallout is very technological but has a low quality of life. The NCR has Vault City and probably San Francisco now which means they have some pretty advanced technology under their belt, House has an army of robots, and even the Legion uses stealth boys, engineering, metal working etc. All of that isn't to say that things couldn't be better in different areas mainly that many technological advances need to be restored (like cars...) and many need to be more wide spread (like electricity). That's why I'm using Maslow instead because technology in itself doesn't produce a good quality of life.
The NCR.
In the long term the NCR is doomed because of two basic facts. A. Their society is almost anti-conservation and B. All the conservation in the world won't produce oil and other non-renewable materials. One thing to be remembered is that the reason the Fallout world is so [censored] up is because there weren't enough resources for enough people, that hasn't changed. Most mines are empty, and there is almost no oil for the taking. That said the only way for the NCR to survive is rapid growth and assimilation, the kind of which we saw in New Vegas, and yet because of it's democratic and bureaucratic nature it's unlikely such an expansion can happen past the Mojave.
This backed up by NCR characters in the game. Hanlon says the NCR has run out of water. The OSI scientist says it's only 10 years until they hit a food crisis. The NCR Gold reserves have been destroyed putting it's currency on the path of devaluation. The NCR is rebuilding pre war america without pre war resources. Even IF they did hold the Dam it wouldn't matter. They need more land for grazing, more mouths to feed, more water for both and Lake Mead and the Colorado is only a temporary fix. There won't be a Hoover Dam around every corner and large scale industrialization is impossible because of a lack of most pre war materials we used for industrialization. The NCR is doomed no matter what.
The Legion.
Ignoring the unethical nature of the Legion let's assume they take the Mojave, now what? Besides a NCR that will continually throw whatever troops and Rangers it can muster at it in the Mojave the outlook is grim for the Legion. The Legion will fall for lack of manpower. Using the information we have about the Legion in NV only and not speculation they aren't strong enough to push into California past the Mojave Outpost, Legate would be the new Caesar by the Second Battle and Legate doesn't trust the Centurions running things back east. From what we've learned the campaign to take the Four States Commonwealth has just ended, and that the Denver campaign almost broke the Legion. Legate believes the Mojave is much like Denver but this time it's even worse with the Mojave. Lastly Sawyer notes the Legion needs to constantly break down members of other groups to survive.
Now with all of that information the fate of the Legion becomes clear. At this point it would be more so a matter of a consolidation of the empire rather than expansion because of two factors from above: The NCR and controlling the Four States. There is some force/s in the Four States that required over 30 years of campaigning to conquer. And even if the Four States remain secure the NCR will still divert resources and probably run some sort of covert operation against it using the Veteran Rangers, a style of campaign that Boone shows can be very effective.
With consolidation, not expansion the goal things start looking worse for the Legion. The Legion spy units would be cast aside due to Legate's hatred of their "woman like tactics" and so to goes their information networks along with the ability to hurt enemies covertly. VR's and Mercs from the NCR continue to assassinate Centurions and higher level Legionaries. The Mojave continues to become fortified draining resources from the Four States. The Generals/Centurions back east have less men and weapons to work with, leading to problems once again in the Four States. The day it falls apart would probably be the day a General back east defies one of Legate's commands or something of the like.
House
House could rule over the Mojave much longer than the decade I predict for the NCR and a few decades at best for the Legion, but lack of resources still remain. The average wastelander in House's Mojave would probably be richer and better fed than most of the NCR's citizens if only because of a lower tax burden and probably lower rates for getting power from the Hoover Dam. The Strip, Freeside and Westside would all be on the level of the NCR's better cities easily.
But even if House is able to sustain this standard of living it would only for as long as the NCR remains a unit. Further this standard of living is still much lower than that of 2051 with a lack of cars, air conditioning, etc etc. The basic fact is that House also doesn't have the means for industrialization, nor does the game indicate such. In the House ending he simply continues on having Vegas be a tourist trap. Vegas and the Mojave under House get to bypass the NCR problem of lack of food and water because the population is much lower than the NCR's but it doesn't bypass the problem of lack of nonrenewable resources.
The key difference between House and the NCR is that because of the lack of democratic and political rights the many will be able to die to keep the Mojave together. The difference between House and the Legion is that is robot army keeps him safe from all threats internal and external. But that's not to say the quality of life in the Mojave will ever reach 2051. Vegas will be something like Vault City or San Francisco under House but Vegas will remain little more than an echo compared to the past.
Independent
Even under the best case scenario (the robot army was kept, peace with the NCR, peaceful Legion retreat and a young courier) all that's basically been done is an House ending without the ability to go on forever into the foreseeable future. It's unlikely the courier could redo House's living forever bit given House couldn't even go back into his chamber if it's opened. That said an authoritarian regime could in theory with the robot army perpetuate itself in power but then for the average Mojave citizen the quality of life would come nowhere near 2051 and it still runs into the lack of nonrenewable issue.
A democratic nation like the NCR would work for a time but I have doubts as to whether or not a Mojave nation could expand rapidly enough to escape the bureaucratic problems of the NCR in it's expansion. In truth probably not.
Conclusions
Short of some type of post scarcity creating device or machine the Fallout world will never return to it's height of 2051, although that's not to say the world can't be made into some better than the period between 2052 and 2077. In the end though the Mojave and all of the Fallout civilizations will fall and history will go on. And with every generation of civilizations in America that rise and fall and war with each other, the chance of post scarcity or a 2051 standard of living world is reduced.
What are your thoughts on the matter? Do you think any faction can bring a 2051 quality of life to the average wastelander? If so how? And if no factions can restore the Mojave to a 2051 level, which factions comes closest?