Schweinsteiger is not a defensive midfielder.Hes a central midfielder that is used as box to box with very good offensive and defensive abilities.Thiago is clearly a central midfielder but more attacking minded than others he can play in attacking midfield position but simply he is not needed there, as you say he didn't have a regular role here and Gotze Kroos are better players.Pep has shown that he will play 4-1-4-1 but had sometimes played 4-3-3 we don't know for certain which he will be using.If its 4-1-4-1 he will rotate mostly with Schweini for him to get rested,if its 4-3-3 he will rotate both in DM and central midfield position.Under Heynckes, Bastian was irreplaceable indeed, but Pep might value other qualities in his single pivot player. Götze and Kroos are both superior AMC, regardless of their nationality. Thiago is vers hyped at the moment but remember, he was a squad player during his time at Barca, the team that got schooled by Bayern over two legs last season. He wont get a lot more playing time at Bayern than he did at Barca, if any.
Another thing we don't know for sure is if Martinez will play in defence.