I wonder what happened about a decade ago.Oh yeah! Fantasy games, RPGs and shows entered the main stream.
You're assuming that this growth is entirely due to 5e's quality, rather than the fact that D&D, and other traditionally non-main stream games entered the main stream at about that time. Had 2e or 3e been released 10 years ago, it would have seen a huge increase from that as well.
I'm assuming that the TTRPG market is relatively competitive. That when 5E was released, it had not been the top selling RPG for a couple of years, but quickly bounced back. That there's no inherent reason for D&D's growth over other systems.
But primarily that if it wasn't decent quality that some other TTRPG would have risen to the top. It's not hard to find alternatives, it's a google search away if the game doesn't work for people. It's easy to find games online. Or maybe it's simply impossible to have any objective measurement of people's subjective experiences.
The post is labeled "What is" not "D&D 5E is" for a reason.