I'll be generous. For every $1M that official 5E D&D content earns, I predict:
5E/OGL-compatible materials from other publishers will earn about $50K
non-5E, non-OGL materials will earn about $15K.
system-neutral or universal materials (like battle maps, minis, dice, VTTs) will earn about $100K.
This is going purely off of my own purchases, a quick browse of Kickstarter, a handful of Google searches, and my "Grade the ___ System" surveys I've been doing here on ENWorld. I don't work in the game industry, I'm not an economist, and none of my sources are scientific, though...so take all of this with a grain of salt.