What about leaving GSX level 1 for FSX and level 2 to P3Dv4?
Nobody will be forced to buy the Level 2 expansion. It's an EXPANSION precisely for that reason.
And, Level 2 is still technically compatible with FSX (although some features won't appear in FSX, like the dynamic jetway numbers, which use the native P3D4 PDK) and, if one doesn't goo to high with settings and/or doesn't use a memory-hungry airplane or a very detailed scenery, it WILL run without crashes so, it wouldn't be right if we blocked FSX, just because SOME FSX users might get an OOM.
That's why there's a Trial to begin with: if you use FSX, we strongly suggest TRYING the expansion with your configuration before purchasing it, something that is always best to do in any case.