I have DB but not DB pro so cant really say much on that one.
I can say however that in my humble opinion if FPSC is moved forward to future versions and it meets with current and some forecast expectations, if hopefully the promotion of the product and unknown buyers potential is good then it will continue to grow in stature and the question wont arise.
We all need to do what we can to ensure the early versions are a success and promote the product. Making good games with it and making sure they are well promoted in various professional and non professional circles is probably a winning way to do this as well as of course promoting FPSC itself.
I mean that as well as the large potential mass market interest user base that may be generated - if promoted in the correct circles then I would suggest there may be a good number of more active peole in the game making community that would like to give it a chance. Certainly in the indie arena - theres not another product currently that can do what FPSC hopefully will allow small developers to do - and thats ship a title of some worth. Theres a lot of dissatisfied indie engine users out there and they are a fair potential market also that needs targeting - That includes all the DB and DB Pro users.
Of course I dont know whether its either posiible or likely in any case but I guess the products may have a benefit one to the other - perhaps they, their content or even the products can be merged to some degree.
Its possible that FPSC may be of more benefit to DB or DB Pro users than the other way around - FPSC will certainly I think be of more benefit to me than any of the other two products.