Doesn't matter much since this seems to be not really true anyway...

That's not what's wrong with it.

There has been almost no definitive progress in fundamental physics in the past 50-100 years.

40-50 years, maybe. 100 years is greatly overstating it - for example, the unification of all forces except gravity dates to the 70s.

That's exaggerated. The Schrödinger equation was published in 1926 and tons of particles have been discovered in the decades after. But most would agree that progress has been slow in the past 50 years, with the discovery of dark energy 30 years ago being one of the most significant ones.

That's just the way science and technology works. You get the largest gains at the beginning as you're picking the low hanging fruit.

You could say the same of chemistry too. Or even about industrialization in general.