If Vettel had won those 3 races and given a Merc a run for their money a few times in a season he would have been happy.
Hah. You really think a driver that has won four WDC's on the trot to be happy with "giving another car a run for their money a few times in a season"? Right
And even if that - they never gave Mercedes a run for their money! In Canada the Mercs were broken and hurting, in Hungary Hamilton had to start from the back and Rosberg was compromised due to SC timing, and I don't think I have to remind anyone what happened in Spa.
The situation at RBR was dire and not looking up anytime soon. Fast forward three years, they're still not close to contending for titles.
There was no promise that Ferrari would be back to winning ways
In F1 nothing is certain. Using that to dismiss a change of teams is useless, as Red Bull coming good would probably be as likely as Ferrari coming good. Unless you somehow think Ferrari is in a perennial losing position which will never subside.
The majority of top drivers in the last 15 years all left for the same reasons, they either left for a better package or they fell out in some way. If Seb had won those races he would have been number 1 at RBR, happier for getting great results - instead he didn't and wasn't happy. A persons attitude towards everything changes and he wasn't happy at being beaten by Ric. Who would? You've won a few titles, also dominated your team mate for a few years and a young rookie comes along (sounds familiar?) and beats you.
Nobody said he was happy because he was being beaten. He can both have been unhappy for being beaten, AND unhappy because he wasn't seeing anything happen at RBR anytime soon. See my first sentence of this reply - do you honestly think a reigning 4x WDC would be happy to fight for whatever leftovers of Mercedes there were? With a PU in the car that was a proper POS and didn't look to become good anytime soon?
Even if he had won those few races because Mercedes or its drivers completely dropped the ball - even if he had beaten Ricciardo, in the end he would still have been in a car that wasn't going to be great for the next few years. So why not take the chance on his dream team where a seat became available and possibly would be locked again for years to come after that since free seats at Ferrari don't tend to come around too often?
Are you really going to tell me that my opinion isn't plausible?
In my view the idea that he would have been happy just for beating Ricciardo, while having a car with a POS engine that likely wasn't going to be great for a few years to come, THAT is what's implausible here.
If you are a rookie in F1, your prime target is to learn and beat your teammate, and win races.
If you have won races, your prime target is to win more and win the WDC.
If you have won one or more WDC's, your prime target is to win more WDC's. Sure you still want to beat your teammate, but staying at a team that won't allow you to win WDC's just because you want to beat a teammate? Yeah, no.