Well, it's a massive hit to Fighter's consistency. If you have a battlemaster, it'll gut a lot of his available utility in a turn. Additionally, you aren't always going to be fighting at level 20, and won't always have 4 attacks, meaning the 2* modifier wouldn't always work; It also doesn't take into account bonus action and reaction attacks, as well as attacks granted by buffs like Haste. It's also a massive hit to the Great Weapon Master feat, since it lowers the amount of opportunities you have in a turn to get a critical hit (also worth noting, it could also mean more wasted overkill damage).
Generally speaking, cutting down the number of attacks is strictly a nerf, and a heavy one at that, even if the damage doesn't supposedly change.
If you don't like the idea of attacking a lot of times, play a Barbarian or Paladin. Or hell, even a Rogue.