Not at all Cesc will not be happy just to be a back up player and neither would Gundogan.
You cant just have two world class players in every position, it doesnt work.
The problem wasn't that Thiago's not a starter. The problem was that we didn't rotate well and he wasn't played enough. As long as he gets enough playing time, I think Gundogan wouldn't mind being a backup, especially considering that Xavi's aging and he'll be a starter in a few seasons.
As for Cesc, he's been benched for most of the second half of this season and doesn't seem to mind. There have been a few rumors of him going, but it looks like he wants to stay. As long as he gets enough playing time, I don't think he'll mind either.
Cesc and Thiago/Gundogan are different types of players. Use Cesc as the Iniesta backup and Thiago/Gundogan as the Xavi backup. That way, they won't get in each other's way when it comes to playing time. If we rotate well, I don't see what the problem is.
And the fact of the matter is, Xavi's not getting any younger. We need a player like Gundogan to replace him in the future if Thiago leaves.