That logic falls to pieces though when you consider it’s possible to bypass the character limit by deleting/restoring old characters, meaning they can and do store more than 50 characters on their end.
Before we see actual negative server maintenance/statistics/costs as a result of allowing 1 more character per account, I really think this is a pointless angle to look at this from. Should we get a blue post stating this would be the limiting factor, then that’s that of course. But so far the only indication of trouble has been that tampering with 15+ years old code in the game’s engine is “scary”.
Splitting the subscription fee into tiers, like you suggest, would then send us down yet another path of pay-to-win, and I sincerely hope they do not do this, ever.