I can't remember for sure now, but I don't remember Fallout 1 or 2 having level caps either. At the least, I don't remember ever hitting a level cap before I'd run through all the content and finished the game.
I think the scaling of the leveling compared to the scope of the game is a more important factor, really. To me, what really matters is - if I go through all the content in Fallout 4 in a playthrough (do all the side quests I can find, explore the map and all the locations, finish the main quest, kind of taking my time but not really grinding or meta-gaming too much) what level is that going to put me at and what's my character's stats going to look like?
To my personal tastes, I'd kind of like to be at a point where I'm ready to go complete the Main Quest and by then have mastered most of the areas that were really important to me and still be sort of thinking "oh, but if I gain just one more level there's still this other Perk I'd like to get..." Ideally, if I then went and did another playthrough making different choices, those two characters wouldn't have the same stats.
If after I finish up with all the major adventuring and wanted to keep playing that character, I think not having a level cap and being able to max every single stat all the way would be great. By then I'd mostly be just playing with the sandbox features, maybe tracking down a couple last side quests or locations I'd missed. I'd have done all the important stuff, my character's "campaign" would be basically done, in a narrative sense - but it's still fun to advance a character and if I'm still having fun playing that character then I may as well keep advancing, even if just for the sheer sake of gaining a level.
I'd (ideally) have made all my important choices in the game, making the manner in which I'd advanced my character impactful and individualized (maxing out END and all it's Perks when I'd played through the entire quest as an END 3 Scientist makes for a different experience than reaching that same point when you're only halfway through the game.) If I then want to keep playing after the game is "done" and keep leveling my character to higher levels and MOAT, then I think it's the best of both worlds.
My worry would be in where the balance falls - if I kind of stop caring about leveling up because I've run out Perks I really wanted for that specific character before I've completed the MQ, then I'll be a tad disappointed. Otherwise I think it's great. I mean, there are quite a bit of Perks (especially if we're counting all their ranks) so I could foresee being able to attain a fairly high level before "over-leveling" would start to become that much of a problem. If that "sweet spot" hits right around the time I'm ready to "finish" the game, then I think that would be awesomesauce.