Because the goal of a beta should be to discover new or hidden issues not to ship known ones to millions of users all over the world.

I think it's still not good enough. And while WP doesn't use semantic versioning, I think that for a big change like this one, it could wait one more version to iron out the glaring issues and make sure the general userbase gets a smooth experience.

Even in marketing compaigns, it would be easier to "sell" a WordPress 6.0 than a WordPress 5.8 or 5.9, so yes, "6.0" also makes for a nice, round launch number.

On the other hand, selling and buying into a beta is a big no, no. Casual users wouldn't want the hassle and professional users would hate the repercussions. Forcing it on all of them only reinforces what the closed hosted competeing platforms were talking about in their marketing campaigns (one of which the Tavern was talking about a week ago).