These people exiting Wizards of the Coast mean nothing to Dungeons & Dragons. We've seen the same thing happen time and time again (especially after new core book releases)... and if D&D is still as popular and is played as often as it is right now even after layoffs following 2E, 3E, 4E, 5E, etc... these layoffs and exits will not result in anything different. It will still be a popular game... 'Dungeons & Dragons' will still be the genericized name non-gamer people use in place of 'roleplaying game' (just as 'Q-Tip' and 'Dumpster' are used for cotton swabs and waste containers)... and new people will step in to try D&D out in the future at the same time older players might step out to try other games.
There's really nothing to see here.
It doesn't mean D&D is in danger of dying or going away or the sky is falling or anything. It will remain a popular game.
However, two of the three architects of the most successful and popular version of D&D in the game's history - guys who have overseen the actual game during its meteoric rise over the past decade - going to work for a competitor does not mean "nothing" in terms of industry news. It's a big deal.