It’s not just Discourse. My husband is a web developer, and a lot of his work is updating old sites that use old code. And it’s not about selling new products. The system he works in (drupal) is open source and free to use. It’s about security and resiliency. Old software is full of security holes that modern malware targets, and also, doesn’t always play nicely with new operating systems that expect more from it.
Discourse is large and complex, and my guess is it’s easier and safer for them to impose minimum standards on the browsers than to not do so.