Non-Windows 10 & 11 have a collective market share of less than 3% of just desktop windows and desktop windows likely is less than 50% of all access to this board. What’s more damning is that the market share of non-Win 10 & 11 appears to be very stable over time which means Discourse doesn’t gain much by dragging this out another year or two because it won’t substantially decrease the number of people it affects. They might as well rip the bandaid off now.
What you’re not seeing is the collective drag on software development speed from supporting a wider range of platforms. Every software development team constantly has to balance the tradeoff between supporting more users vs building more features. The costs are real, even if they’re invisible to the consumer.