I'm not convinced MLS teams want to become a finishing school for Europe to a point where they can operate transfers at a profit, I think that's a general supporters misinterpretation of what's happening and I agree it'd be nearly impossible to do.
I think the goal for some clubs is just spend big money on players with resale value (as opposed to aging European players with limited resale value) so that they can make enough money on it to justify higher transfer expenses which means more money invested and raising the standard of the teams/league. It doesn't have to be profitable to make sense, the idea is focusing on players with resale value gives us more flexibility if things go wrong which derisks the signings significantly. Soteldo is the perfect example, guy did not work out but we weren't saddled with a contract and were able to leverage him to bring in another hopefully top level player. You wouldn't be able to do that if a top aging European player on a mega money contract didn't work as nobody else would want the contract never mind give you a transfer fee (or player equivalent) to take it
It's not about profit, but just about more options. Sign a player with resale value and if it works out incredibly well you make a huge profit (this is an outlier, and not the goal, it's a bonus if it happens). If he flops, you can offload the contract easily at worst and at best actually get some money back. Middling outcomes are you get a great MLS player for a long time (as they're younger) or they do okay and you either recoup the transfer fee, or most of it, or potentially swap them for someone else. Even beyond that you can get a few good years and some money back
The outcomes for the aging star are much lower. Best case scenario the guys a megastar for a few years (this, again, is an outlier) and then leaves on a free/retires after a huge investment. Middling scenario is the guys very good, justifies the expense and sells tickets before moving on likely on a free. Worst case is a disaster, though, you've an unhappy player and you've a contract nobody will touch which you either eat or you pay them several million dollars just to not play for you
Both have merits. I'm not complaining about TFC, as a fan I'll take Insigne over any signing in league history frankly, but I think people need to see the nuace of the re-sale value approach. It's not 'turn a profit or it has failed'