Wizards of the Coast sold over $31 million in print units across the 18-month period between May 2017 and October 2018, according to Bookstat. So that would give annual sales of about $22 million. Digital adds only $1m, so $23 total. Total RPG sales (ICV2 data) is $65 million.
So, the raw numbers show that D&D has 33% of sales, a vastly lower proportion than the portion of online games being played on roll20 -- hence my statement that the proportion of money spent on 5e compared to the overall industry is much lower than the proportion of online games that are 5e.
Now these are pretty vague numbers -- Bookstat might be reporting sales outside the US/Canada whereas the ICV2 data is US/Canada only, so 33% may be too high (I have to depend on Forbes' reporting of Bookstat info as I am not a member). On the other hand the total ICV2 data likely includes a ton of stuff which is system agnostic (miniatures, maps, dice bags, etc.). Heck Dwarven forge is probably a noticeable part of that
But even if you assume that a third of all gamer sales is system agnostic (which seems high to me) we're still only seeing D&D at about the 50% mark --much lower than online proportions.