Whne the '80s turbo cars had unlimited boost and fuel they didn't need to rev much more than 10-12k. And they had substantially more power than today's engines.
All the examples of the turbo cars in the 80's had one thing in common. They were near their max RPM at 12K or whatever similar number you prefer. They had mechanical valve springs. They did not have the option of increasing RPM so of course they didn't.
To get more power they simply cranked up the boost. This does not mean that is the only way to run a turbo engine, it just means it is the only choice they had.
When they cranked up the boost on the turbo engines they did get more power but they sacrificed reliability by placing tremendous loads on the bottom end. Those 80's turbo cars had huge problems with reliability. In fact some of the spectacular engines making wonderful horsepower well over 1000 were qualifying engines that were designed to only run for five laps or even less. In those days when the engine went south you could just throw on another lump and get in the next race. These days the engines have to last longer, through qualifying and the race for about 3.8 races on average. So just piling on the boost at 10,500 RPM or less might not work very well for the second, third or fourth race.
Also, since the good old 80's, mostly during the V10/V8 years, there have been huge gains made in bearing technology and heat handling so the friction losses that used to be such a big deal back then are not so much now. The bearings in the 20K V8's are actually tiny compared to the bearings in the 12K turbo engines of the 80's.
I think everybody agrees that the reason they currently run the engines where they do now is because of the regulations that force everybody toward 10,500 RPM. That is where the regulations allow the maximum fuel flow rate of 100 l/hr. You don't get any more fuel to run higher RPM and you get less fuel if you run lower RPM. It is the fuel flow limit that keeps them from going higher or lower. Obviously the engines do not run at a fixed RPM, they run over a range of RPMs when they shift gears etc. So in order to get the most power you need to be near the highest allowable fuel flow rate so the range of RPMs to get the most power will be near 10,500, say 9,500 to 12,000. Pick you favorite numbers here too. It will depend on your engineers and the gear ratio choices etc.
Remi Taffin says - "It's important to say it's based on the regulations, because they set 15,000 as a maximum, but the fuel-flow limitation means the maximum we're running, whether it's a Ferrari, a Mercedes, or a Renault, is 12,000 and at the end of the straight it could be 10 or 11,000."
I definitely believe if they removed the fuel limits they would run the engines at higher RPM.