In a lot of ways, in the NFL anyway, current defenses are more reactive than proactive. Rules that increased defensive holding calls, decreased contact with receivers, decreased an ability to hit a QB or WR have made playing defense a much more reactive game than it was 25 years ago. Back then blitzing defenses with tight man coverage that included a lot of contact with receivers at the line, could proactively shut down offenses no matter the style of offense played. Now, almost exclusively, the defense has to react to everything being done offensively. Zone blitzs were created as a reactionary result of some of these rule changes.
This makes for a lot of the great QBs, and even many not so great ones, dictating how the game will be played since so much power for the outcome of games have been put into their hands. Offense dictates the flow and style of a game now, not the defense. defenses no longer shut down offenses but contain them. The rules have been manipulated in such a manner as to ensure higher scoring games and offensive players being protected from injury so that they can be healthier for a longer season to guarantee higher scoring games all year and get higher television ratings.
Playing great proactive defense has been de-emphasized in the name of weekly ratings and so, the MVPs of the NFL are now and for the foreseeable future,going to be offensive players and probably QBs
Keuchly is an outstanding linebacker but all he is doing is racking up tackles. For every play that a score does not happen, a tackle will be made by someone. Racking up tackles isn't nearly as impressive a thing as what LT was doing on the field in the 80's. He was sacking QBs at an alarming rate back when QBs rarely dropped back to pass 30 times a game. The shotgun was not an everyday style of play back then. 300 yd passing games were not an every week occurrence and yet Taylor was sacking 15-20 QBs every year for a period of time then.
Stats such as forced fumbles, tackles, and tackles for loss were not kept in LT's days but if they were I am sure you would see a measurable difference between a true defensive MVP year and Keuchly's year this year. LT was a game and rule changer. Like players in the NFL and other sports(Chamberlain, Orr, Gretzky, Ruth, Koufax, Unitas), Taylor changed the way the game was played. He changed the rules of the game. That's what an NFL MVP who is on defense is. That person is a game changer.
That's not Luke Keuchly, as much as I love him as a player. Heck, I don't think there exists a game changing defensive player in the NFL right now. And for that reason, I wouldn't expect an NFL MVP being from the defensive side of the ball anytime soon.