In my experience running long-term games, as long as you're not consistently running marathon 8-hour sessions, XP from hitting alignment/bond and saying yes to the three important questions at the end of session pulls ahead of XP awarded on failed rolls.
In general things swing a little to one side or the other - if people are regularly failing rolls they'll get more failure XP but accomplish fewer of their session objectives.
The problem with a 10th-and-enough-XP-for-11th character is that you can't wave XP in front of them for any reason at all. Somebody who wants to keep going after that point is probably sufficiently motivated by accomplishing something in the game world that they want to succeed at the adventure, but if you've ever been using any informal mechanics like "mark XP to follow the Siren's Song or defy danger to do anything else" those lose their teeth.
Which is why they suggest taking an apprentice if you want your main character to keep going.
So, a couple questions to ask this 10th-level thief.
Have they literally never met a single NPC they wanted to bring along? That's what taking an apprentice is for. They don't have to be anything related to a thief or get taught thief skills at all. "Being an adventurer" is enough of a craft to pass on.
What's it going to take to get them out of the adventuring business? Ask it even if they take an apprentice. Adventuring is not a safe profession. At any time you can run out of hit points and die. "Retiring" does not mean "and the GM ships you off to a farm upstate where you can skulk around all day and backstab rabbits". They'll still exist, as someone in the world, and as the GM you're not going to play them different or ax them out of spite. They'll still be doing the things they want to do, just under your judgement. Heck, write them the occasional love letter after significant breaks in the action.