First and foremost, from me this gets a FUCK YES!!!!!
...no unit ever becomes obselete)
Well, that's not true, since Queens and Devourers are apparently useless (and most games never get as far as things like Arbiters, Carriers etc.). But that's besides the point since the rest of your post is spot-on.
Basically, if they change a lot, people will bitch and whine about it; if they don't change much, people will bitch and whine about it. I'm just happy it's finally happening, and it's not like I even ever played online anyway (I actually really enjoy SC's single player campaigns...).
People complaining that the Protoss got loads of cool new shit but the other races got gypped make me lol, since it's obvious they just haven't revealed much about the others yet. From what's been shown, it looks like the Protoss's "thing" is going to be massive mobility (aircraft that can turn into pylons and the ability to warp in new units to ANYWHERE in the psionic matrix).
Also, what we've been shown looks overpowered somewhat (hello Mothership) compared to the kind of stuff we're used to. Balance is a top priority with Blizzard, so I can only assume that the other two races will get similarly OTT new units and abilities. Possibly kind of like how Guilty Gear supposedly achieves balance (from what I've read, basically just by making EVERY character broken).