Operating income (mostly subscriptions and microtransactions) from 1 April 2021 through 30 December 2022 for the entire Digital Entertainment division (MMO, HD Games, Mobile/Browser games) was 49.4 billion Yen (about $425 million dollars). Divide by 9 and you get an average of $47 million per month. With 35.8 million players at $15/month the operating income for the MMO group alone would be $537 million per month, not $47 million per month. Your estimate is off by at least a factor of 10. So no, there aren't 35.8 million subscribers per month. (For reference, WoW at its peak had something like 12 million subscribers.)
There are 37 million 'subscribers' listed in the last set of population charts I've seen. I've also seen 25 million 'registered users', which is probably more accurate, since 'registered users' do not have to maintain active subscriptions. SE reports the number of players based on accounts, not current subscriptions.
Live population statistics appear to hover around 2 million since the WoW explosion. One percent of 2 million active players would be 20,000 players.I find 2 million actively subscribed players to be a better number.
So 20,000 players contributing $15/month would be $300,000 per month, based on an income of $30 million per month for the entire actively subscribed user base. Not chump change, but faaaaaaar from you $2 million per month guesstimate.
There is no way for Square Enix to determine whether that population of players is 20,000 or 40,000 or 2,000. You can create an argument using the fuzzy math that both you and I have used here, but you need solid proof of the numbers, rather than mere conjecture.
It is far easier for the Linux community to do what it has always done for games that are not officially supported by a company -- put their own wrappers around the code and fix any problems that come up themselves. I know it's not what you want to hear, but that's the way things work.
It is entirely possible that SE developers have already heard about this, and they may make small changes to accommodate players in this position, but there is no way you can demand those changes until SE adds official support for Steam Deck and/or Linux. Until now, you've been working with a duct-taped workaround rather than playing on a supported platform.