Epyo the Great posted...
Why does Valve keep heavies powerful as much as they can? Because they are central to the game in a way no other class is (besides medic). They're sort of the tank class. Valve does what they have to do to get people to play heavy--make it powerful, because it's not interesting enough to play by default. That's why medic and heavy got the first class updates.
Err...no? Heavy is not that central to the game. Medic obviously is central to the game. Soldier I would list as central (if you listen to the designer commentary, every single map is designed with rocket jump height in mind, and their knockback is a big deal, and a lot of HPs are designed with rocket clip size in mind). Engineer is central to the game (above a certain number of people, engies could have teleporters and nothing else and still be worth using, and despite the high-pop focus of the class, they're still sometimes used in 6v6 on defence. Not to mention an entire class has a weapon that only works on engies). And maybe Demo is central to the game (even ignoring how nuts they are in 6v6, all kinds of stuff was added to the game just to handle stickies. Bullets kill them and airblast knocks them away. Oh, and they're "the blindfire class").
Heavy/Scout/Sniper are the next tier down. "They kill things, and they have the most range/HP/mobility." With Sniper probably the class out of those three that the most stuff is designed around (like class HP and maps). The game still works fine without them, though; if one of them were removed from the game, something else would fill the niche (like without Sniper, Demo becomes the go-to long-range class).
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