One reason why this probably won't be happening is that WWE isn't the least bit concerned about TNA and what's going on there in general. Why should WWE care? It's not as if TNA has ever been any sort of genuine threat to take the #1 spot away nor is the TNA brand have anywhere near the level of recognition or history that WCW had. As much as some have always lauded TNA as being in competition with WWE, it's never really been entirely accurate. Oh sure, TNA is technically a rival wrestling company, and is therefore competition by default. Unlike WCW, however, TNA isn't, nor has it ever been, taking money out of Vince's pocket. After all, WWE hasn't made any majorly radical alterations to its format or business practices in response to anything TNA has done whereas the Attitude Era came about almost as an act of desperation on Vince's part to fight back against the rise of WCW's popularity. In all honesty, TNA being the #2 wrestling promotion in the United States has only come about by default.
If WWE truly had any interest in "putting the final nail in TNA's coffin", one way they'd have tried to go about it was to relentlessly hound top TNA stars and raid its roster with big money offers to jump ship once their deals with TNA expired or for said stars to ask for their release. Years back, allegedly, WWE was interested in bringing in guys like AJ Styles, Matt Morgan, Bobby Roode and James Storm but they were all happy in TNA. Now they're either in their late 30s or starting to approach their late 30s and WWE's developmental program has improved by leaps & bounds since they last reached out to top TNA talent. As a result, the feeling among WWE officials is that they don't need to bring in former TNA talent as none of them, aside from guys who were already made into stars by WWE and/or WCW, are going to make that much of an impact. When it comes down to the numbers, TNA is still drawing in the very low 1s, often drawing under that these days, house shows regularly continue to draw well under 1,000 fans, ppv buys continue to reportedly be only a small fraction of what WWE ppvs do and that's how it's been for years now. WWE Main Event usually out draws Impact Wrestling and, for all intents and purposes, its overall format is almost identical to that of Superstars. WWE reasons if none of these wrestlers are a draw in TNA, then there's no real incentive to go after them at this point.
As far as NXT goes, while it's technically under the WWE umbrella, it generally operates within the confines of its own "universe" as WWE and TNA do. As a result, for the most part, it does seem like its a separate promotion. It has a different feel to it than either WWE or TNA and is packed with fresh talent that most audiences are unfamiliar with. If WWE is ultimately able to sign a US television deal for NXT, then there's a strong probability that it will take over the #2 spot from TNA even though, as I said, it's still technically under WWE's control.
