In that regard I'm inclined to believe there is some sort of stagnation. Though not necessarily at the fault of the community or researchers. It could very well be a mix of what you said about problems being harder that we have, between 1900 - 2000 picked all the 'low hanging fruit' in physics, and that we now need lots of patient, 'baby step' type improvements to get to a new era where the technology is powerful enough to make big leaps again.
Kind of like how Deep Learning research stagnated due to lack of data volume and processing power.
This is my perspective, but I think it's totally up for debate and I'm keen to hear different opinions.