It’s too long.
I’ve played lots of board games, several longer than Monopoly, yet somehow Monopoly feels too long to me, too. It’s the only board game that I know of that has many mods added by players for generations, but the rules advise against it because they can make the game longer.
Here’s my crazy theory: there is no perfect length, there is only a bad fun/time ratio. Monopoly is too long, compared to…