Rest assured, I don't think they're ever going to retire or remove it. IMVU (or what's left of it) would die in an instant if they got rid of Classic completely. No if, but, or maybe about it; it would die then and there in about a week. The only way this wouldn't be the case is if they replace it with a straight upgrade. Not the desktop beta or Next, an entirely new client that works identically to Classic and/or does even more. I also don't see that happening however, so I think Classic will be around as long as IMVU is. Also if it wasn't for the Mafias and 3rd party clients, I actually think they'd of reduced or even tried to stop updates a while ago. As-is they're doing the bare minimum for a better user experience and bug fixes and most of their efforts into 'security'.
I'd also like to think if anything ever did happen to Classic, as unlikely as that is, DM and Don would have something to fill the void and allow people to keep using Classic, or something like it. For what it's worth, I only use Classic. I think Next, Beta and Mobile are unanimously awful.
A small part of me wishes things would evolve to compete with SL, both graphically and in terms of movement and control, but it really is wishful thinking. I feel any chance of that happening is lone gone, along with the potential of IMVU developing a new client that isn't dogwater. (Their golden years and high income days are long gone.)