Interesting way to think about the hypothetical. Quite a few players you could say the same about. Couple that come to mind immediately -
The Ajax legend that went to Barcelona and became a legend there as well. If you did the flip then:
Cruyuff, the Barcelona legend who retired in Ajax. Have to wonder why Barcelona let go of such a fine player.
It's actually a great comparison to the Ronaldo scenario since Cruyuff did go back to Ajax into his older years for a few seasons.
Nedved, the Juventus legend who retired in Lazio. Have to wonder why Juventus let go of such a fine player.
Ashley Cole, the Chelsea legend who retired in Arsenal. Have to wonder why Chelsea let go of such a fine player.
Buffon, the Juventus legend who retired in Parma. Have to wonder why Juventus let go of such a fine player.
I think ultimately it comes down to how petty the fans are. It's like how some people hold bitter resentment for an ex and can't just accept that it didn't work out long term, be happy for the good times and be glad that they're doing well in their new life.
With Ronaldo especially, unlike say Ashley Cole, he went to a different league and he got Sir Alex's blessing and even gave him an extra season of incredible output. All the nasty fuss came from Spain - you see it now w Mbappe and Barca's president - they're a bunch of self serving clowns. From Ronaldo's accomplishments and what it meant for the club at that time he is a legend. The only reason not to think so is that pettiness and personal dislike for his superstar/fame, most of which came from Madrid.