How did Michael not deserve being the Don? He was the only one who stepped up to the plate destroying his father's enemies while sacrificing his own innocence at the same time.

Godfather part 2: Michael built his family enterprise into a multi million dollar empire, at the same time competeing with enimies out to destroy him.

I don't think tessio or Clemenza or even Tom would be able to handle/bring The Corleone Family up (not even close) to where Michael brought it.

p.s.
Sorry if my grammar sucks.