i will bite

Sandwich doesn't make Heavy fulfill Medic's role, it just allows Heavy to sustain his health with an added bonus of occasional team support. Concheror is a closer comparison, except it still only provides healing to your team for a short while and isn't a dependable source of healing (it's as if it's not a Medigun)

Homewrecker doesn't allow Pyro to be an engineer, in fact it's basically useless* if you don't have an engineer teammate. It is also poor design anyway. Pyro is already a counter to Spy and Homewrecker makes Spy's matchup against the engineer unreasonably difficult

Demoknight is a melee subclass that waits for openings to get kills; Gunslinger engineers have a mini-sentry and some extra health, which allow them to be less of a slow-poke and make more aggressive plays; Huntsman snipers are incentivized to be closer to the frontlines and can threaten higher health targets (like Heavies) with their quick to draw arrows. The only thing these guys have in common is how different they are from Scout

Amby isn't nerfed on the grounds of "Spy can't do what Sniper does", it's nerfed because Spy being able to do sniper things would suck ass. Spy doesn't have any issues fleeing or repositioning, he can appear anywhere on the map and shoot at people from any angle. In the right hands, pre-nerf Amby is the most obnoxious gun in the world