Actually yeah, 100% your words. I assume you have it from good authority that it's not Sergachev that by himself changed his style when confronted with how hard it is to play in the NHL.
Let's find a middle ground. Therrien is not in the develloping business, he's in the "win now" business like all coaches.
Sergachev is not stupid - he sees that. He's perfectly conscious about his own mistakes so he tries to play safe, to avoid being the one costing his team points. Playing "safe" means not playing his style.
So, indeed, it's probably Therrien's attitude that triggered the change in style - however, it's probably the same attitude 90% of the coaches in the NHL would have. When you want to get into the playoffs you can't afford losing points because of a rookie. Would not help him either in the long term - guilt and such.
Bottom line - the NHL in a competitive team (as in not a bottom feeder Leafs-like) is only good for developpement when you are NHL ready. Otherwise it will change you or break you and we don't want Sergachev to go through that.
When Subban started playing in the NHL he was ready. Not the case for Tinordi or Beaulieu.