Money not being an issue can still happen: See Valve, which have a money printing machine in Steam, along with a few long living, lucrative games. Work on Half Life 3 never really stopped , and we had a 13 year wait between Half Life Alyx and Half Life 2, episode 2. Sources say that the internal standards for a Half Life 3 are so high, it led to many restarts and a lot of work being thrown away.

But the real reason we don't have games that run as long as Deus Ex is that the cost of reworking what was already there have exploded since those days. If you are predicting a 3 year release cycle(not unheard of to this day at large publishers), and you are delayed by 2 extra years, the hardware you are targeting is different, and the amount of art you have to upgrade or completely remake is crazy compared to Deus Ex. With teams being far larger than they were to meet a 3 year plan, marshaling a modern AAA game team to rework large parts of a game has little to do with managing to do the same with a team from the late 20th century. A linear increase in team size gets you a geometric increase in coordination costs.

That said, we still have small teams that might work on a game for a very long time, but they have to aim far lower technically, or in scope. You can make a beautiful, shorter game with a small team, like Outer Wilds. You can also make a 2d game, and tweak your game to perfection, like Stardew Valley. At those sizes, saying "when it's done", and letting yourself change direction and scope is fine. But try that with the Red Dead Redemption 2 team, trying to be at the forefront of everything. Without a far more rigid plan, you can go into a Duke Nukem Forever spiral (and DNF was a far smaller team!)