That's a pretty big if, but yes. There is nothing wrong with the idea.
The problem is:
No it can't be judged separately from what PD has delivered to this point when the execution of the idea is what matters. It's not a recent track record that you are asking people to overlook. It is a consistent, overarching one that dates back all the way to the first GT game with only one example of a strong deviation away from it; and one that only happens to be at its worst in GT5 rather than a new thing.
Perhaps there is some way that such an idea can be implemented into the series without making the UI even worse than it has traditionally been (and I'd like to see some game try it and do it well, because even Auto Modellista had incredibly convoluted menu design despite being built around such customization; and that's the closest a game has gotten to the OP's idea). But I don't care enough for the concept itself (at least not compared to having a simple freeview system to let you look at the cars closely like that in Forza 4 or pre-Criterion NFS games) to give PD the benefit of the doubt that they could actually do it without just adding on to the UI problems their games already have.
The way I see it, there's two extremes to the way PD could move forward in terms of the interface, and how that affects the idea presented:
1. Start from scratch; there's no better time to look at the bigger picture and see what else they could bring in or leave out etc. and design it from the start to make the most of it all. No penalty for including the feature.
2. Add it into the existing UI; it's already a mess and this isn't going to change a thing (we already have the framework for custom backgrounds loading in the, er, background from GT5

, too). No penalty for including the feature.
Complain about the existing interface at your leisure, it really does need improving, but it is by no means a reason not to do something like the OP is suggesting. It's a pretty weak argument against it altogether, actually, given it's just a
backdrop that the UI sits on top of. All those UI improvements you talked of could still be made anyway - just stopping the redundant caching and uncaching and phoning-home would work wonders, but we really need something totally new.
As it stands, I'd rather have something like this
and a crappy UI than just the crappy UI.