yeah you bring up some good points. it's definitely a nebulous area. honestly i do find the steam key metadata interesting. one problem with it is that not all games released on steam and sold on itch provide a steam key on purchase.
Viewing post in discovering mainstream games on itch.io
Of course not. But at least we know that they are on steam, if they are on that list.
Another list might be the top seller list of paid games. Or the iOS list.
The game list has some 890k entries. The iOS list only 4.5k, paid only 24k, steam keys only 2.4k
I estimate that any successful games (outside of itch) are on one or more of those lists. The games with minimum price 0 but a bonus tier with a price offering the full game might be missing. They might be found under regular top sellers. Example would be Doki Doki Literature Club. That one is on steam (for free), is pay what you want here but sells a fan pack, if you paid more than a certain amount.