It might be a mistake to let TC walk but Cuban had his own reasons to make that decision, mostly financial incentives. He had been one of the biggest spenders the years prior and maybe he thought it was time to fold things up when the primary objective was already achieved.
Them Mavs wouldn't have won another ring after 2011, with or without TC. They would've stayed in the le contention with TC on board, yes, but things wouldn't have turned better for them than the post-08 celtics. They would continue to "contend" for a few more years but their championship count would remain at 1, then it would come the rebuilding process and they would be tied to mediocrity for years to come, with their best players gone (like the Celtics).
Them mavs still has a franchise player in the form of Dirk, and an excellent coaching staff led by Carlisle. They've refreshed/replenished their squad with le-chasing veterans who're just as hungry as them 2011' mavs. It's not like we could replicate 2011 with the current squad but we will stay relevant and challenge the best teams like we did through the 2000s. We're sending a message to the rest of the league that we're not going to tank, we had every reason to tank the past season with Dirk injured but we didn't. we kept fighting for a playoff spot till the last week of regular season, and even when we were already out of the race we still played as hard as we could to avoid a losing record, which we did. Tank for Wiggins? Sorry but them Mavs have too much self-respect to ever do that.