The data is freely available to anyone paying attention. You can just add up a bunch of publicly available numbers and get a pretty breathtaking total of revenue generated by rpgs that are not D&D. I think folks generally don't care though. Why should they?
To me, and I suspect Mike would agree, if you want to understand the hobby business, revenue is not the only thing you should be thinking about. For one thing, D&D makes a lot more money than 5E does. 5E is book sales and there's no growth there and there never will be. You just need to hang around and wait for some kid to do something alarming or wait for a global pandemic and suddenly there's an influx of new users. But that's not a growth strategy.
But what are people actually playing? That seems pretty important to me. And I think there are good and useful ways to find out! They used to publish, and I assume they still do, a list of all the RPG event tables booked at GenCon and which game system they were booked for and that is very revealing.
To me, that is a real indicator of market share.
Anyway, anyone's who's been around long enough remembers a time when RPGs were doing well, lots of independent companies able to make a go of it, while D&D was doing not-well.