At the current time FSX is the largest market.
What evidence you have to support this data ? I hope you don't confuse "number of users" with "number of users that purchase add-ons".
I believe Aerosoft did a poll BEFORE P3D V4 was even announced, and they had somewhat like a 50% split between P3D and FSX, and this doesn't even take into account how much these users buy. I guess the same poll made today, with P3D V4 on the market, will be quite different.
I think you should have given it at least another year before deciding to cut off FSX users
Which is exactly what we are doing.
KSDF, which will be released shortly, will still be for FSX. It's the next sceneries we'll do after that will be for P3D V4 only, which right now are confirmed to be KPIT, LSFB and the new KORD. None of them will likely arrive before at least one year so, we are giving you exactly what you are asking: about a year in advance to prepare yourself.