You’re 100% correct that the overall (Windows + MSFS) memory usage can and often is in the 16-20 GB range, but what we’re speaking about here is DWM.exe, a Windows background process, that sometimes decides to leak an additional 500mb to 7 GB of ram. So in your case, it might show 15 GB in use, but only 8.3 would be doing anything productive.
I agree with you that the page file should not need to be edited, but this was at the direction of MS support. Apparently the more memory you put in a computer (going from 16gb to 32gb, for example), the page file will actually increase, which is somewhat counterintuitive. I found that when mine was set to “system managed,” it was actually at 13 GB, which is way overkill. Even though the “system recommended” value was around 4GB. I’m probably never going to max out my 32gb of RAM, so I set it to the recommended value. No idea why the system managed setting was so much higher. A benefit is that I gained back 9 GB of SSD space.