3 of those young players are already gone again and 1 is always injured. Obviously Schürrle was a bad buy, Toprak on a free wasn't that stupid even though he isn't good enough to solve Dortmund's issues in defense.
None of those transfers helped to build a strong team longterm if we ignore the financial side of things. It isn't totally unreasonable to want some experienced quality players. I'm sure Tuchel would have prefered to spend some money on better established players than Schürrle and Toprak, but simply wasn't allowed to.
Outside of the money made with Dembele, Dortmund's signings weren't particularly great overall for quite some time now.
Toprak for free? Dortmund got him for €12m (not even that bad a deal imho) and that was only because they waited a year for his release clause. Tuchel wanted them to sign him straight up when Leverkusen's asking price was €25m.
Schürrle was a disastrous buy, especially since Tuchel didn't even seem to have a clear plan for him and quickly cast him aside.
Bellarabi is sitting the bench at Leverkusen now. That's around €80m Tuchel would've liked to spend on bench material.
Dembele was Dortmund's mvp last season, at times carried them in the cup (especially in the final two matches) and was sold for a mind blowing profit.
Guerreiro is one of the most talented players in the squad and despite some injuries played an important role under Tuchel.
Merino barely got any minutes under Tuchel and when they wanted to loan him out last winter Tuchel blocked that because he had a plan for him by which he apparently meant around 150 minutes of play time. After which Merino decided a change of scenery might better for his career.