As much as I'd like Thelin in charge now so he can begin his magic at Hampden, it makes sense for him to wait until summer.
Big move, uprooting his life from Sweden to Aberdeen, being thrown straight into a huge game v Celtic and working with players he won't know. It didn't do Glass, Goodwin or Warnock any good in the short-term (or long term) taking over a struggling team towards the end of the season. He could have a few early defeats and have fans doubting him early.
Makes sense to move when he's been able to watch from afar and get to know a bit about the team, and come in summer ready to rock and roll.
By all accounts he's very "boring", not one to criticise referees or make outlandish statements. But Henrik Larsson was the same and until now he's the most successful Swede in Scottish football, so I'll take it.