From my understanding project purity was to purify the Tidal Basin and the Potomac River in one shot. Broken steel changed it all and made it so only the water that goes into the Jefferson Memorial gets cleaned and the BoS give it out in oil drums. Which makes the Enclaves plan totally useless.
I wouldn't say "totally useless" at all really. Here's why I think that:
I think the virus plan was more dangerous and would have purified an area larger than we assume. Its possible (and im just winging it here) that when the virus was released into the water supply via the purifer (maybe there is something with the purifer that magnifies the viruses killing range/power, after all it uses a GECK, which Bethedsa made into almost a god-in-a-case) that a combination of evaporation/water cycle/atmospheric winds would have meant the virus entered into the atmosphere and poisoned a good sized portion of the east coast. And if the virus was able to enter and contaimate a part of the Atlantic Ocean (closest to DC) via the potomac, then prehaps something would have happend with that as well (evaporation, entrance in atmposphere). Whatever the case, when you put the virus into the purifier, it says the Enclave "wins" on the East coast, that the Enclave was now allowed to thrive, which means Edens plan was a success.
Anywho, I'm going off topic, personally I think it doesn't really matter what the number of the next game is, or whether New Vegas should be considered Fallout 4 or not. In short:
I don't care, so long as the game is good and worthy of the Fallout title it has.
This