I honestly think the new era of gaming(shoot!! Shoot!, bang, bang, boom!!!!!, etc) is mostly what damaged the popularity of Tekken and other fighting games. I do not think dumbing down or streamlining the next Tekken release will automatically send gamers flocking to it like bees to honey or is a resolution to the problem. Fighting games unfortunately seem like they are becoming a niche genre once again and I think the sales for each are indicative of that. TTT2 sales were definitely admirable for a fighting game, but it's the lowest selling Tekken of all time which is a travesty because of all of the unprecedented effort, content and time that went into the game. Namco imo really gave true Tekken fans the definitive Tekken experience with TTT2. Tekken is, has and will most likely be popular with the hardcore crowd but sadly, the game just didn't appeal to casuals or was labeled as too complicated and the fact that shooters, sports and other genres dominate all and they have more accessibility. Casuals to my knowledge like fast, easy access to games, flashiness and even handholding. If a game is too complex or requires some or substantial effort, there is a chance they will drop it and move on. Nothing wrong with that, but that is reality. Honestly, I think introducing the tag team mechanics also turned off some gamers. Heh, I know some people who went back to Tekken 6 or Revolution because of them. I think if Tekken 7 went back to the one vs one mechanics, it could bring back some fans. I thought they were overwhelming at first, but ended up really liking them and getting fairly accustomed to them quick.
Also, another potential scenario is some Tekken fans have moved on and play other types of games that require less commitment and time. Tekken has been out for approximately 20 years, right? I've been playing Tekken since Tekken 1 in the arcades and I am still playing it today. Most people I converse with say, I use to play Tekken back in the day when Tekken 3 or Tekken 5 was popular. There is a new Tekken game? Oh, ok. I didn't know. I just play COD and Fifa now. Not really into fighters anymore or they think their reflexes aren't what they used to be.
I think Tekken Revolution was actually a good idea for the casual Tekken/fighting game fans and for a more laid back experience, but I do not want them tampering with traditional Tekken where it loses it's identity and depth. They could potentially lose their diehard fanbase in trying to appeal to people who do not care for fighters in the first place. I say leave Tekken complicated....er...deep and let the fans decide if they still want to play and invest time with the franchise and let the others play Tekken Revolution 2 or whatever. Going back to 1 vs 1 mechanics will probably cut down on some of the complications, so that could be a start.
This is of course, only my opinion.