I don't really see the reason to lament. For one, just because LaMahieu has played well in Colorado doesn't mean he would have done the same had he stayed with the cubs. And besides that, had the rest of the events that played out did, he still has no place on the team going forward. Even if you look at that as not being something that was guaranteed to happen, Watkins at the time was higher rated, you had Alcantara who was higher rated, you had Baez who was higher rated and as we've seen hasn't stayed at SS. I'm not certain but I think Lake was a infield possibility too at that point as I think he was still playing 3B.
And even if we ignore that and say what is what is today's value of LaMahieu the answer is still not much. That's not to shit on him as a player as he's a decent second tier starter but no one is going to give you much for him and with what the cubs have in terms of prospects he isn't a long term solution. I doubt he'd even have played over Valbuena who turn his playing time into decent value in returning Fowler.
So, regardless I don't really see that the cubs lost anything. Additionally, it's easy to say man Ian Stewart is shit now. But keep in mind this is the exact tact the front office took trying to rehab Arrieta, Strop, Valbuena and various others with spectacular results. Stewart was BA's #38 prospect in 2008, #46 in 2007, #16 in 2006, #4 in 2005, #57 in 2004. This was prior to having Mike Olt. It was prior to having Kris Bryant. Stewart was going to be 27 and was one year removed from hitting .256/.338/.443. That season the cubs had all of Stewart, Valbuena and Vitters as 3B options. If someone wants to argue they shouldn't have let ARam walk then fine. That's probably the right guy to fill that problem. However, if you take him out of the equation for whatever reason it's an entirely reasonable move they made.
Either way, i really see no reason to feel bad that LaMahieu isn't part of the cubs anymore.