It would probably be useful to compile a list of times in the past that coming out the other side of the bull's horn was worth it. If you're trying to find a common thread.
What immediately comes to mind as an obvious example is Newtonian physics. There was a period in the history of science where it looked like we had figured out almost everything worth knowing in this field. That turned out not to be the case in a big way. There were clues that there might be a deeper, more general theory in the inconsistencies in observational data at the time and it seems like this would be a good place to start.
What fields are there that seem like they are mostly figured out but with a few nagging inconsistencies? Continuing the physics theme, the standard model does a heck of a job but there is still dark matter/energy and gravity to figure out. It's clear from the number of top level minds devoted to studying these things that people already think this horn is worth bulldozing through though. It's not that people don't think it's worth digging, it's just really hard.
Maybe there are fields that are similarly saturated theory wise but have inconsistencies that aren't being thought about a lot?
The key is confirmed experimental results that are other than predicted by established theory. When theory is very well established, there is a tendency to out-of-hand dismiss contradictory results as probable errors. Sometimes that "theory of error" is accepted without the errors ever being identified. This especially can happen if there is mixed success in confirmation, which can happen when a phenomenon is not understood and is difficult to set up.
Nuclear physics is such a field, where quantum mechanics is incredibly successful at making accur...
Some time ago I learned of the metaphor of 'digging the bull's horn'. This might sound a little strange, since horns are mostly hollow, but imagine a bull's horn used to store black powder. In the beginning the work is easy and you can scoop out a lot powder with very little effort. As you dig down, though, each scoop yields less powder as you dig into the narrow part of the horn until the only way you can get out more powder is to turn the horn over a dump it out.
It's often the same way with learning. When you start out in a subject there is a lot to be learned (both in quantity of material you have not yet seen and in quantity of benefits you have to gain from the information), but as you dig deeper into a subject the useful insights come less often or are more limited in scope. Eventually you dig down so far that the only way to learn more is to discover new things that no one has yet learned (to stretch the metaphor, you have to add your own powder back to dig out).
It's useful to know that you're digging the bull's horn when learning because, unless you really enjoy a subject or have some reason to believe that contributing to it is worthwhile, you can know in advance that most of the really valuable insights you'll gain will come early on. If you want to benefit from knowing about as much stuff as possible, you'll often want to stop actively pursuing a subject unless you want to make a career out of it.
But, for a few subjects, this isn't true. Sometimes, as you continue to learn the last few hard things that don't seem to provide big, broadly-useful insights, you manage to accumulate a critical level of knowledge about the subject that opens up a whole new world of insights to you that were previously hidden. To push the metaphor, you eventually dig so deep that you come out the other side to find a huge pile of powder.
The Way seems to be one of those subjects you can dig past the end of: there are some people who have mastered The Way to such an extent that they have access to a huge range of benefits not available to those still digging the horn. But when it comes to other subjects, how do you know? Great insights could be hiding beyond currently obscure fields of study because no one has bothered to dig deep enough. Aside from having clear examples of people who came out the other side to give us reason to believe it's worth while to deep really deep on some subjects, is there any way we can make a good prediction about what subjects may be worth digging to the end of the bull's horn?