I think that the picture is complex, and I'm sure that the two (currently) top comments by David Roberts and Brendan McKay, although providing arguments in apparently opposite directions, are both spot on.

I believe there's also another phenomenon which is worth mentioning: we generally feel more comfortable in assigning the "breakthrough" mark to works that have aged well. Without the kind of pedigree that only historic evolution can give, it's more difficult to get consensus about how large the impact of a result will be. Of course there are exceptions, like if a celebrated conjecture is proven, but for evaluating the breakthrough character of new ideas, frameworks, connections...and so on, time is usually needed.

So my two cents: in the following centuries, a non-negligible set of results, which currently are somewhat lost in the clouds, will be considered breakthroughs, as it often happened in the history of math.