City have never won back-to-back PL titles, plus there are several other clubs who are credible title challengers, all of whom have kept all of their best players. Bit different to Italy.
Would Ronaldo even improve City that much, considering they've got Aguero? Would he score that many more goals than Aguero? Last season Aguero scored 30 in 39, would Ronaldo do much, much better than this? I doubt it.
I think your view of the tax issue is blinkered and naive, to say the least. Both Messi and Ronaldo have paid vast amounts of tax, and are also hugely wealthy. Neither of them has anything to gain by deliberately avoiding tax, and neither of them are directly involved in their own tax affairs. Equally, neither of them really deserves any sympathy regarding their financial circumstances, or life in general. If Ronaldo chooses to leave, it is purely a self-interested decision, not some rational reaction to maltreatment.
The guy is a multi, multi-millionaire, who lives his life in a golden cage, his ego continually pandered to (pretty much the same as Messi). To suggest that he left Real due to poor treatment is laughable.