If you are suggesting that the current ICEs are not as advanced, or are less stressed, I think you are wrong.
The first point about stresses is that the V10s and V8s had strokes of around 40mm. The current engines have a stroke of about 53mm. That has a lot to do with the rpm discrepancy.
The V6s also have boost. Possibly as much as 4 atm manifold air pressure, whereas the non turbos have about 1. The compression ratio in the cylinder head is comparable to that of the V10s/V8s, maybe even higher (limited to 18:1). The in cylinder pressures of the V6s are very much higher than in the V10s. In terms of engine stresses the cylinder pressures probably make the V6s more highly loaded than the V10s/V8s were.
As for making less power, yes they do. About 10% less power using about 49% less fuel. That doesn't sound easy to me*.
* The easiest way to make more power is to put in more fuel. But more fuel requires more air. For a turbo engine you can wind up the boost, but for a NA engine it means adding capacity or adding rpm. Since capacity was limited, rpm was the only way to get more air into the engine. Had the FIA not restricted the engines to 10 cylinders there may have been a move to V12s, as these allow smaller piston diameters and strokes but greater piston area. Smaller pistons and strokes enable greater rpm, more air, more fuel and more power.
I am not suggesting the current ICE are not as advanced. They are probably more advanced particularly the efficiency and also since time marches on.
I am suggesting they are less stressed because they are. That is by design and by regulation.
They do make less power but again that is by design and regulation. They could easily make significantly more power than the V10's ever did simply by giving them more fuel and more boost but the regulations do not want that.
The main reason the V6T's are stressed less than the V10's is because of the operating range. The V6T's operate at about a maximum of 11K RPM while the V10's were pushing 19K RPM.
I used 18K RPM for these calculations because that was more common for them than 19K RPM.
I used 11K RPM for the V6T even though their fuel formula maxes out at 10.5K RPM they frequently hit 11K before shifting.
The V6T, with a bore of 80 and stroke of 53 has:
mean piston speed: 19.45 m/s
max piston speed: 31.57 m/s
max piston acceleration: 88,698 m/s2
The V10, with a bore of 95 and stroke of 42.3
mean piston speed: 25.38 m/s
max piston speed: 40.71 m/s
max piston acceleration: 181,458 m/s2
These piston speeds and piston accelerations of the V10 are nearly all human engineers could handle at the time. They could probably handle more now but they don't have to develop that technology since the V6T's don't approach those levels.
As a comparison
2018 Mustang Shelby GT350 at 7.5K RPM has a max piston acceleration of 72,029 m/s2
Top fuel dragster making 11,000 hp at 8,200 RPM has a max piston acceleration of 131,503 m/s2 - (some builders use new pistons for each quarter mile run)
Two of many other areas where the RPM operating range causes huge stresses are obviously the main bearings and the valve train.