I would dispute this:
The 20th century streak of breakthroughs in science, especially physics and engineering, has ended, and relatively little (in comparison) has come from some the 21st century's major research paths such as the search for dark matter/energy or for a theory of quantum gravity.
There have been a huge number of breakthroughs in the last 25 years. They've been in, and fueled by Information Science and technology and accelerated by the advances in communications technology.
We haven't found a complete theory of Quantum gravity, but aided by supercomputers and we did successfully detect gravity waves. The paper publishing this result has more than 1000 authors from all over the planet acknowledging the scale of the collaboration that went in to achieving that result.
Do you know how much less efficient it would have been for 1000 academics from all around the world to collaborate on a project of that scale in 1910? 1970? even since 1999 things like practically free video conference calls anywhere anytime and storing petabytes of data, let alone sending petabytes of data across the world in a matter of hours have became possible.
New theories on dark matter/energy are spawned weekly from tweaking the variables in immensely complex computer simultations of the development of the universe until they find something that fits observed reality (or as close as the model can come).
The theories, technology and infrastructure that enables these have been largely developed and are continuing to develop at an accelerating pace over the last 25 years. I don't believe that STEM has stagnated. It's just advancing in different directions than it did 100 years ago.