I think people forget that there are levels to that 'backwards compatibility'. Things like mesh and morphs and skeletal rigs are one thing; this is where things can be rendered useless. Shaders, Lighting, these are really on a throw away level. I started using Vue because of the more flexible lighting system it had (among other reasons). Learning new methods of setting up ligths, or using shader nodes is not  in the 'It Cost Me Money$$!!!' category; you can recreate old light sets with new lights. Replace shaders with other shaders. Most of what's out there shader-wise for Poser is either free, or was part of something else. They could replace that system and the learning curve would not be that great. If they used something like OSL, there would be tons of pre made shaders for people to just plug in and go.

That said yes, there would be a lot of whining about the favored 11 year old light set not working at one click anymore. More power means you have to bother to learn to use new features of the program. Nothing is free. Ever.