It's a tradeoff they've made. Allowing the plugin store to be a free-for-all creates a very low barrier for entry, and the marketplace is thriving and well populated as a result. They've added metrics you can use to assess quality, in the form of compatibility reporting, user reviews, stars, etc.
Then start making the quality rules apply. Or produce code quality ratings for plugins and display them for people to make decisions on.
Making sure that WP can keep working with lower-quality plugins in the name of 'compatibility' just ensures that WP is, at best, a mixed-bag.