As @RGJ explained, we use feature-detection to ensure that browsers support all features which Discourse depends on. If you circumvent that feature-detection, then some things will be broken.
Those things might not be noticeable to you, or might not apply to certain forums/themes. But they are real. We don’t prevent use of certain browsers for no reason.
Allowing people to use Discourse in a browser with some missing features would lead to a lot of confusion and support load. Being clear about requirements upfront is the user-experience we’ve chosen. For those older browsers, forum content is still readable to anonymous users in the basic-html view.
We put a lot of effort into supporting older browsers, especially where the upgrade path is less easy (looking at you… Safari). But we have to draw the line somewhere.