Aircraft compatibility is not really a problem in fact.
As long as aircraft makers are able to produce new aircraft for this new sim, it is absolutely NOT a problem if you can't use the old planes from FSX or FS9.
No, the real problem is that Dovetail is making it difficult for addon makers to produce anything. First of all, the SDK are not available yet. And then, their pricing policy is not attractive at all.
Because of this, even though the product is still in alpha stage, most addon makers have already shown their lack of confidence or motivation to produce anything for FSW.
And now, P3Dv4 has killed FSW.
No matter what FSW will try to bring, P3Dv4 is what the community was looking for. "Community" encapsulate both the simmers AND the addon makers of course.
Just look at what is happening: not only do the simple (no external gauges) addon work out of the box, but ALL the addon makers are already updating their products for P3Dv4 at full speed.
The community has made the choice already. This choice is P3Dv4 and XPlane11.
(And P3Dv3 for those who want to keep using FS9 planes).
FSW is left alone in the dark for now. Unless they bring some unbelievable features and performance improvements (I mean, we can always dream, right?...
), that sim is just as dead as MS Flight.