I think it's easy to underestimate the extent to which the greatest minds have also turned to every weird permutation you could think of. Personally I figure we'll probably find some deeper theory just since the Standard Model still has its issues and gravity isn't rolled in yet, but there's every reason to think that this problem is hard, not just untried. Much of the most interesting work in the last century has been ruling things out.

There are lots of nifty ideas that are explored until insurmountable holes are found in them. The two main nonstandard lines of thought that have had any real progress are decoherence, which I'd call a success, and string theory, which I'll avoid rating because I have string-theorist friends but am not a liar. There are plenty of others, and maybe one will bear fruit, but honestly when the next big break comes it'll probably be really obvious.

Why would you assume it would be obvious? Copernicus's heliocentric model was wrong in many ways: he assumed uninform speed, that the sun was the center of the universe, that orbits were circular, assumed the necessity of epicycles, and so on. His main victory was simply that from our perspective of complete knowledge we can now safely say he was less-wrong than the geocentric theories of the time; there's also the David vs Goliath narrative which is emotionally satisfying.

If one wanted to cast doubt upon, if not "debunk", his idea from the science of the times, it would not have been difficult to do so (see: Tycho Brahe). And that was on an issue that was likely some orders of magnitude less complex than the one we may be facing today. The implication of that being that the "right" answer may initially seem to have more holes than swiss cheese. I think it's very safe to say that relativity, undoubtedly the monolith to which all scientists aspire, was an exception and not the rule in the march of discovery, in its reception/clarity.

Good point. I guess all the great minds of this time see occupied with how to maximise ad clicks.