Will 5e be the "last" edition of D&D?
Maybe.
It could very well be, as far as we define editions. If they do a 6th Edition or revision, it might be more akin to the 1e/2e split where the content is largely backwards compatible. 6th Edition could be a smaller tweak of an edition, with less substantial changes than we saw in 3.5e. Almost more of a repackaging of the rules with more attention paid to presenting the rules in an easy, accessible format.
I don't see nearly as much pressure on the rules to change or be updated like I saw in 3e or 4e. Fewer people thinking of cracking apart the base system and remaking sweeping sections of the rules. More adjusting smaller subsections, like certain classes.
But there's probably no way 6e will ever be as popular or high selling as 5e is.
Which is fine. D&D does have a big pope/ small pope thing. 1e/ 3e/ 5e are big while 2e/4e were less well received. It makes sense that 6e will be more of a filler era that maintains the company while people keep playing 5e but no longer need books until 7e comes and can capture the market again.
And D&D is doing better than ever. It had its best sales year ever in 2017, and 2018 is reportedly even better than that. Can it do that for a third year? Maybe. Probably not. There's a finite number of players who can join the game. I imagine PHB sales will start to slip in 2019 and sales of accessories will be lower. Sales will still be great: still probably better than sales during in 3e and 4e. But just lower.
I love 5e, but unless the spring and fall book are amazing, I almost have all the content I need. Eventually sales might drop enough that a reboot or relaunch is needed...
