I don't think so. It's strictly more work than designing a good UI for a single device which is hard enough, and usually more work than designing multiple UIs for different devices no matter.
A smartphone, iPad, and ultrawide monitor demand fundamentally different UX considerations. Generalizing across all of them, and in one codebase, demands a certain expertise or else everyone would be doing it.
I don't think it's a question of expertise; it's a question of effort (and cost). So it's a management issue; are we going to put the effort in to support desktops, or are we going to settle for a crap website? Because a website that doesn't support desktop is a crap website.
A smartphone, iPad, and ultrawide monitor demand fundamentally different UX considerations. Generalizing across all of them, and in one codebase, demands a certain expertise or else everyone would be doing it.