Historically speaking 30FPS was always FS nirvana back in the day (FS 2000/2002). I wasn’t on board at the PC launch of FS2020, but came on board for XBox release, but am now on PC. The XBox release was targetted for 30FPS locked. Now that was for a very specific set of hardware, but I’ve read articles suggesting 30FPS is where it’s meant to be at. I bet the devs don’t want to publish that as it isn’t headline grabbing in the gaming world. No doubt the devs have left it open so people can try for more if they have the hardware, but there are trade offs involved in letting the sim do that. There should be a options selector that targets fluidity so 30FPS,60Hz → High ‘let sim decide’ FPS like a smoothness v performance setting. My first reaction to hearing about 30FPS was do I really want to be there? and why if I have a good card should I be there? but after switching over to 30FPS/60Hz the fluidity over the entire sim (not just above the clouds) has kept me there and I rarely check my framerates (they are locked at 30) and rarely/never change any graphics settings as they are otimised. CPU never limited and GPU comfortable at 70% (leaving a little extra headroom for anything I encounter along the way). The switching to 30fps/60hz frees up resources, reduces stutters and creates that fluidity. It makes me think that that is where it really is meant to be running at.
Generally, you are right. It’s better to have lower consistent framerates than higher framerates with frametimes all over the place. That creates microstutters that are extremely displeasing.
However, if I start at an airport with 30-35 fps and return to that airport after an hour of flying, I expect to get 30-35fps again not 12 or 14. That’s the issue. The Minupulators take 15ms instead of 3ms which slows down the main thread and makes the whole simulation CPU-bound where it wasn’t before. That’s a bug and nothing else. And there’s nothing in the settings that can optimize or compensate for that. That’s simply a bug in the engine with (supposedly) garbage collection not working and slowing down a process with lots of data which is not thrown out when not used anymore.
I get your whole point about frametime consistency and I agree. But the framerate should be the same after an hour of flying and returning to the same airport again. And the manipulators should take the same time to calculate.
Off topic: I was on board since the MS-DOS launch of Flight SImulator II in 1984. 
The more I think about it, my issue might be related to After playing a few hours, FPS drops from 40 to 5FPS
Now that is a separate issue entirely. I think they have a worklaround last time I looked, but no sim fix unfortunately. Devs can’t replicate on their H/W (see last Dev Q&A). If it’s after a few hrs of flying then yes it may well be that issue, and it’s been around for ages (since at least last summer).
If it isn’t that issue, someone else found a complete reinstall of FS after SU8 helped get their FPS back to where it was. I’m not sure why. It’s a pain to contemplete doing that with every SU though.
I haven’t found any issues post SU8 other than having to do a NVidia driver update after initially getting 2 CTD’s. Maybe I’m lucky. I have no idea why a few others are seeing stutters and frame rate drops all of a sudden (seems like quite a few are). Possibly just unoptimised systems and the way they are reacting to the latest sim update patches. You also can’t perhaps rule out management or mis management of mod by some users. I’m always very regular with mod update checks, and remove anything that’s not confirmed as compatible.
The problem seems to exist since more than a year: Intermittent Performance Spikes, high time in Manipulators