A flat rate for everyone, applied against everyone’s individual revenue seems fair to me. That way you contribute to the thing proportionally to what you are getting out of it.
It is in fact all about paying for what you USE, rather than own idling. For example, visual studio or creative cloud subscription will cost you even if you are not actively using it. Also, depending on your revenue, the upfront cost of a product or service might vary anywhere from negligible to unbearable. Paying a percentage of revenue means you pay 0$ for things you don’t use, and you never pay more than you get out of it.
I don’t think there is a rational reason to split into big and small apps, if there is something that merits higher taxation, at least in my opinion, is predatory apps that rely heavily on the in-game consumables, pay to win and whatnot - since those, again - in my opinion, don’t add anything practically useful to the end user or society in general, and are basically addiction exploits, so if such apps are allowed at all, they should be subject to significantly higher taxation.
Other than that, it seems only fair to have the same % upkeep fee for everyone - it doesn’t seem correct to effectively punish vendors of popular apps, just because they developed something that is popular with people. Having higher revenue doesn’t implicitly mean higher profit margins, because in many cases, more popular apps also have a larger development team behind them, and more operational expenses. Nothing to put such vendors in a position that justifies charging them more, just on basis of raw revenue. Maybe if it was net profit… but even then - a bad business model can easily drive margin down, and it doesn’t seem like something that outta entitle someone to preferential treatment.
As for certain companies arriving to drawing such an arbitrary distinction recently, I don’t think this move is concerned about fairness in the slightest, it is more of a face saving thing, and the rather absurd “we care about indies” translates to a perfectly expected “we will make a concession on a very small portion of the overall revenue, keep overtaxing the bulk of it”.