He made errors in his time here but it's obviously not a huge indicator for a young player. Every transfer has a push and pull factor, his was no different. Pep was the pull factor that brought him to Bayern specifically, but it didn't push him out of the club.
What did push him out was Tito. Thiago's playing time was erratic and often irrelevant. He AT LEAST deserved the consideration of the manager to play in a few meaningless league games after we had already won the title to keep the buyout from being as low as it was, it would've showed some sort of belief and desire for him to stay. It's myth to say that he had some sort of bratty attitude and wanted more than he deserved while our other players were firing it up on a regular basis. Cesc was having his 2nd half season syndrome, Xavi was overplayed to the point of fatigue and painkillers (which, yes is a big deal) and showed no impressive form of his own. Iniesta was our only consistently awesome midfielder last season.
Thiago himself was 5th in our MOTM thread (3rd in the chart dividing votes by playing time) which is a fairly consensus opinion, check the archives for yourself. Those who said he showed nothing to deserve more playing time last season are fooling themselves. He had brilliant matches vs Athletic, Espanyol, Zaragoza, Deportivo, Getafe, and a few other decent ones. He warranted trust and was fully capable of handling himself apart from some bad moments. All the hate and mention of isolated errors came in the summer when the transfer speculation started.
In the end he didn't even leave because of ONE season of less than desirable playing time partly due to injury. This wasn't a case of him "wanting to be undisputed starter without earning it blah blah blah", he just wanted more faith and effort from the coach to give him a role of importance rather than cast him off as "reserve player" or "4th best". He left because there was little effort from the coach to incorporate or trust him, and that itself is damn obvious. Tito's words: "I can't promise him anymore playing time" and nothing after that to try and get him to stay. He said that despite Xavi continuing to age, despite Thiago being a massive prospect for us, and despite the fact that he did play well. He couldn't even give him enough minutes to keep him at the club ffs! That's horrible management. Pep's words: "Thiago is the only one I want". It doesn't take a Nobel Prize winner to tell which manager had more faith in him, and look how well that's payed off. Thiago's talent was nurtured and now he's shining for Bayern and even with the NT vs. Italy. He's not undisputed starter for Bayern (other than what he's EARNED) and he's constantly saying how happy he is there. This is in a midfield which is more competitive and has much more depth than ours, goes to show how much of an impact coaching had to do in his move.
It was very short-sighted to restrict his playing time off of a pea-brained seniority system that leaves no competition for starting places and just hands it to the veterans. Calling him a reserve shows how the distribution of games was based solely off of reputation and peak quality, not taking into account factors such as fatigue, age, form, player trajectory, etc. which is what any professional management should be doing. After that, it was taken a step further in arrogance with the whole "Can't promise him anything about the future" as if that's a sign of bending to his will. He had earned more trust for the next season, he wasn't just asking for it without working which is what the high and mighty "no one is bigger than the club" mantra would suggest.
Thiago isn't exactly a rooted loyalist but it's a joke to expect that of anyone in modern football (Xavi himself was near a Milan move at one point). He had ambition and a desire to grow, but with Tito and the coaching staff neglecting him so clearly I don't blame him for wanting to find his way somewhere else. It's a shame because Tata would eventually take over and he would've given him trust but that's a matter of hindsight.