I mean, I understand if people disagree, but many rank Tannehill as a top 8-9 QB in dynasty. He's gotten better each year and he just got a lot of new, arguably better, weapons. He's a locked in starter for his team, which is something that isn't true for a decent number of NFL QBs (many of which he already had on his roster!). Even after the RG3 trade, not counting Tannehill his options were Cutler, Foles, Alex Smith, EJ Manuel, and RG3. So that's guys with very tenuous grasps, or no grasp at all, on a starting QB gig (Cutler, Manuel, RG3), and a couple mid to low end QB2s, at best (Foles, Smith). That is a pretty dire set of QBs in a superflex/2QB league. Adding either of the rookies wouldn't really change that, and even if they do become legitimate NFL QBs, that will likely be a few years down the road. There's also no guarantee that they turn out more like Luck or Tannehill himself than like Alex Smith.
To get an established starting QB that is young and projected to be top 10, without giving up an established starting QB, requires giving up some good assets. Without knowing the rest of his team, I expect QB is the biggest weakness. So even if it was an overpay by a bit, it was probably both necessary to make the trade happen and still improved his team overall. Maybe the Tannehill hype has exceeded his real value (not sure, but I don't personally think so; more like Teddy's has!). But if not, and you think he's a 27 year old top 8-9 QB with a nicely upgraded offense and an upwards trajectory, it doesn't seem like any kind of crazy overpay to me.