Four years is a long time away. People need to remember that while D&D is having a moment, it is just a moment and very likely will fade back to "normal" levels, which will mean WotC (or whoever owns it) will want to shakje things up again. None of us know how long streaming is going to be a thing, or if some other game is going to grab the streamers' attention. Imagine of the Critical Role crew decided to engage in a 2 year long campaign using a different rule set. Would the Critters follow them to that game and leave D&D fallow? Or what if the success of the new animated series means fewer and fewer streaming games happen as the cast members and Mercer spend more and more time doing different kinds of work. Will their fans still be playing D&D 6 months after the last Twitch stream?
I dated once to suggest that 5e would eventually start to taper off and a new game, similar to how Vampire & Pathfinder either overtook D&D or came really close, and someone demanded evidence that it would happen and a timeline. Basically berated me so watch out with some people suggesting such a thing. It was scary.