Considering how little PS and XB have talked about the sales of their premium consoles, I'm thinking they must not view them as successful. At least, not big successes. If they though they were, they'd certainly proclaim it loudly to us.
On a personal level, I liked the idea when we first heard about it, and even through the launch of the systems. But, in hindsight, I think I'd prefer that the generations be shorter (maybe 6 years rather than 7) than we get a small bump in the middle and have to wait 7-8 years for the new generation to start. With that said, I think I'll be starting the new generation with an XB (I have two more years on my GP subscription, so I think XB makes more sense for my first purchase), so I'll be watching closely to see whether it makes sense to wait for an upgraded PS5. If it doesn't look like one is coming, I'll probably buy a regular PS5 within 2 years of launch.
Now that I think about it, the fact that I'm sitting an waiting on possible future information, rather than buying a PS5 sooner, is probably a big negative for mid-gen upgrades from a business point of view.