Cue Valve's latest fuck up in TF2 changes .... tinker too much and things will break

Its not the classes that need to change, more game modes are needed IMHO. Different scenarios lead to different class usage and new ways play the same old character.
Not quite with you here. Some of the class updates are really expanding the game and change the gameplay in interesting ways. Combustion burst is a great tool, Natasha IMHO is a good, yet underrated weapon and I also like the Kritzkrieg as an alternative. Weapons like these provide you with different tools to respond to different situations keeping the game more diverse.
That having said some of the updates are nonsense and for example the Backburner was unbalanced in its original form. I also dislike the packaging of the class updates of a complete set of weapons together with ridiculous achievements which achieve actually only to disrupt constructive gameplay for a few weeks.
But you're completely right about new game modes. Nothing to keep a game fresh than a new sandbox to play in

Walker feels the Engineer's sentry guns have become "too binary." They're highly lethal to some players and wholly ineffective against others. Sentry guns are also easily dispatched by skilled players, leaving Engineers with little recourse.
I could be mistaken, of course, but I think there could be a misunderstanding here.
I could imagine that Walker rather meant "classes" instead of "players". Some of the classes are completely helpless against a SG (Scout, medic, partially pyro). And while those classes are not intended to be SG-killers it would be nice to increase their chances a bit. In TFC the scout had a nailgun which was more effective against SGs. Medics and Pyros carried grens.
And to some degree I can see why he makes that remark about the engy. Once the SG has been disabled/destroyed the engy has little to defend against a medium or heavy class.
Again, in TFC the engy had handgrens and the mighty EMP-gren, which where very effective against heavy classes.
(Not suggesting to re-introduce grens to the game, but there sure are ways to enable some classes in more appropiate ways)
I also like to point out that it's a flawed point of view to expect a class to be balanced to what an excellent player can do with it. You have to make compromises so that a (merely) good player can be efficient in that class. An extraordinary player will always exceed that and perhaps make that class look overpowered.