It didn't start before Manning, let's not lie to ourselves. As late a '99 there were times at the RCA dome there was more fans rooting for the other team or with non-Colts jerseys on. Sure the Cardiac Colts of 1995 grabbed people's attention, but it was short lived. Of course the Colts fan base increases when they make the playoffs and even more so when they make a run, but before Manning and really before the team saw success in the early/mid 2000s, the Pacers had the stronger fanbase.
As far as Miller never giving Indy a title, yeah, but it was despite, not in spite of him. He just didn't have a strong enough team around him minus possible the 04/05 team that destroyed itself. The NBA was ridiculously strong back then and even with top players, you needed a good team around that player. Look at the Bulls, Jordan is arguably the GOAT, but he had great roleplayers and an incredible supporting cast. Heck they made it to the Conference Semi-Finals without him in '93.
I do agree with your last two points and as I said, what Manning did long term and his legacy is stronger than Miller's. He put Colts football in the spotlight and possibly saved the Colts from moving.
Yeah CS, what makes RW so cool is that he respects his elders, the greats who came before him, wants to glean as much knowledge from them as he can, & writing that Manning letter was just automatic for him out the admiration RW has regarding Peyton's career, his philanthropy off the field, & how much Manning honors the history of this game which Russell wants to emulate completely himself.