From here, you can continue to sell your engines to developers from all walks of life, not just Purebasic users. Heck, what if your biggest customer base are not Purebasic users? So what. What you will have is a set of tools that you can maintain easily and safe in the knowledge that your chosen development platform will still be here when I am pushing up daisies (and if I don't cut out the barrels of coffee I consume, then that will not be very far away!) Your designers can surely be made to spit out code in a variety of languages/flavours. It may well be that Powerbasic users will continue to be your most profitable customer base, and there is nothing wrong with that. Purebasic DLLs work fine with Powerbasic (just a few tweaks needed to handle strings!

Seems to me that you are not really open to suggestions as to how EzGUI might be able to evolve beyond what it has been throughout it's life, and I understand that. You have clearly invested a hell of a lot of work and time in a product which you are rightly proud of and probably feel that in it's current form still has a lot to offer. I cannot comment upon that because I do not know the product very well.
If you are to stick with EzGUI in it's current form then, well, I think you know better than anyone what your options are. All I can say is that from a Windows point of view, there is nothing EzGUI offers which you cannot do in PureB (and so a port will not be difficult). Once done, you can still support all your existing customers as well as open up new avenues. With each new major release of PureB, the cross-platform capabilities of the language just gets better and better and so, with one eye on the future, you would be in a pretty good position to eventually think about a cross platform version of EzGUI. It's win win as far as I can see.
Having said all this, let me answer the question which I think you perhaps should have asked much earlier : would I buy EzGUI as it is, or if you ported it to PureB? The honest answer is no, but that is only because with the work I do, I am not in need of such a tool. I have been a Windows die hard through and through, but even I am now moving towards cross-platform, for the challenge as much as anything.

Just my 5 cents worth.