Long for a group, 1 or 2 years. Long for a campaign? So far 35 years of Greyhawk, with many different groups. Dozens (note the plural here) of them.
You didn't answer the question asked.
Again, you said most. That is your experience. Players love to see their former incarnations being put to use. That the kingdom they built 20 years ago (real time) be a thing in the campaign world. To see that their character is still a major player in the world event and can be their patron. You mixed up high level with longevity of the campaign and campaign with campaign world. At some point, a campaign can be both a campaign in a campaign.
@Lanefan's campaign, unlike mine has been uninterrupted for quite a long number of years. A free form game like the one you propose does not suit well for such long campaigns. Unless you record every single sessions and adjust the world accordingly, but then, the free form becomes a fixed form campaign. These "exploratory" campaigns are best suited for one shots, from a single game to a campaign lasting one or two years. And my guess is that two years of every week play is a bit stretchy.
It's fun that you start by chastising my about "[my] experience" and then go right into asserting your experience. And continuing to be incorrect about how games other than D&D work.
Familliarity for playing a little bit and understanding that it was not really my cup of tea. But still interested because the concept is nice. So far I have played a lot of game systems that are not D&D. You assumed that I played only D&D. You are dead wrong on that one.
No, I assumed that, since you listed a number, you've played many games that have the same authority structure are D&D -- basically where players only have action declaration authority and feelings authority for their PCs and the GM has everything else. I don't think you've had experience outside of that authority structure because you keep asserting things that would have been shown to be incorrect if you had. Unless, of course, your experience was with others that also didn't get it and tried really hard to play the game the same way they knew how.
And you know how I might know about this? Because, at one time, I also had no such experience, and said many of the same things you're saying here, and didn't understand what I was talking about. I grew up on D&D, played other games that had similar structures, and thought that was how RPGs worked. Took awhile, and really it took my getting totally hooked on the concept of Blades in the Dark to try and figure out how it might work by discarding everything I thought I knew about how it did. So, yeah, when I see people making the same kinds of arguments I did back when I was clueless, and which I know are actually wrong (not opinion wrong) now, it's fairly easy to guess what experience base they have.
Again, familiarity does not make me an expert in the play style you hold so high. The few examples I have lived through were not conclusive for me as I have seen the "I WIN" button too often. Be it free form or in the resource expenditure way of doing it. And again, I did say that the form had its merits but it requires a specific set of players that are not that common.
I run and play 5e, so please don't try to make this a me evangelizing a favored game over other games. When I run 5e, it's with the stock authority structure, I'm just well aware of what it is and how it works and can make some changes sometimes without trouble. I also play other games that don't work at all like D&D. Currently I'm in a Stonetop game that is a riff on Dungeon World. These games feature many of the same tropes as D&D -- fantasy, early iron age, frontier, scary monsters -- but do not play the same way at all. My GM right now has zero idea what's going to happen in the next session, but is literally drooling to find out, and won't be doing any prep because it wouldn't help. We know where the game is now -- my character and another PC are being attacked by some agitators and guards and are in big trouble -- but we don't know where it will go. I might just win the day with an impassioned plea for mercy right off the bat and then we go from there and see what happens next. I might botch that, and we go from there and see what happens next. Dunno. And the town we're in we all just learned a lot about it last session because I made a horrid string of bad rolls and that resulted in the town getting fleshed out in ways I didn't want because that was the dramatic need evinced in play there -- the desire to help this town and solve a few problems. So, since I failed, we found out that the town is more under the sway of dark powers of despair because my character's main thing is hope. Had I not had my string of failures, the town wouldn't be enthralled as it is and we wouldn't be facing what should be a friendly populace but instead is twisted with fear of the outsider. And before someone says "that could happen in D&D," sure, it could, but only if the GM decided so, on their own. In Stonetop, I had the opportunity to set things as I'd like them, failed, and the GM was
required to pay off my PC dramatic needs in the consequences.