I agree with a lot of what you've said. However, there are two absolute untouchables in our squad (on here anyway), and Sergio is one (probably with good reason). There were 3 until Kompany had so many recurring injuries, and some are now realising that we can live without him. I don't think we can currently live without Aguero.
On his day Aguero is the best striker in the division. I think Guardiola wants more from him though, as he's already said. It will be interesting to see if Aguero has it in his locker to do that - he doesn't appear to be the kind of player who won't work at it though, but only time will tell. The fact that, Ian aside, he's our only striker is an issue for us as a club (although I believe Guardiola has said his dream would be to have a team full of midfield players).