I realize that probably none of us know the answer for sure. I'm also not asking specifically about either console. I'm just wondering, how does the gaming industry view the success of the mid-gen upgrade experiment?

It is pretty clear that these upgrades did not extend the length of the generation. Did they lead to greater software sales? Did they get people to buy into an ecosystem that they otherwise would not have?  

On a personal note, I can say that I did not have a PS4 until the Pro launched. I was thinking about skipping PlayStation this generation. But, the Pro gave me a good excuse to get in to PlayStation.  So, at least in this one case, the mid gen upgrade did get someone into the ecosystem that may otherwise not have done so.  I just don't know how many people really did that.  And, considering that companies make little or no money on console sales, it seems like these upgrades wouldn't make sense if they didn't get a good number of people to do like I did.

Anyway, I'm interested in hearing everyone's thoughts.

I think they view them as "another thing we have to test on"

It probably doesn't make anyone more money (other than the people selling the new hardware".

I bought both (pre-ordered, even!) but, to be totally honest, I never really saw them as huge improvements. Not big enough to recommend them over the base systems.

I don't think they expanded the market or anything, either. It was the "fear of missing out" combined with buzz words like 4K and FPS. Games looked better but they were still the same games and thus, not worth the upgrade.

The 3DS kinda an upgrade done right because it did offer a substantial, tangible improvement in both power and functionality (built in right analog nub) but it also hurt the success of certain games by dividing the market.

Not sure how I feel about multiple tiers going forward.

I believe Sony released the Pro because they normally saw people transitioning to the PC about midway into the generation, so this was an effort to keep more players on Playstation.  The only way to measure this would be to compare PS4 software sales to PS2 software sales.  Not sure if it's helping PS4 software sales or not, but I'm sure Sony is tracking this carefully.

We can only hope so.

Tough to say. On the handheld front, Nintendo did this with DSi and New 3DS. I think it helped in those cases. The DSi in particular was a success because it added a camera and a digital games (DSiWare) store. But I think those platforms would've continued to sell regardless, so a mid-gen specs boost wasn't needed.
The PS4 Pro is basically the best 1080p console on the market. Even if you have a 4K TV, the resolution is dynamic in a lot of games. Sony dropped the ball not including a 4K Blu-ray drive. If the Xbox One S can have a 4K Blu-ray drive and be very affordable, so can PlayStation. Xbox One X delivers a much better (though not uncompromised) 4K experience. The Xbox One X did not seem to help Xbox One much besides adding 5-7 million more units sold, and the bragging rights of the most powerful console on the market.
I suppose the answer to whether or not mid-gen upgrades would be helpful for the Series X and PS5 are very much up for debate. We'll have to see what developers can squeeze out of the consoles during their lifetimes. Let's just say a game like GTA V would've been pretty unthinkable on the Xbox 360 and PS3 at launch, yet they made them work in 2013.
Circling back to Nintendo, there are debates of whether or not Switch will get a "New" or "Pro" version. If Switch 2 doesn't come out until 2024, I'm very sure they will. But if it's out in 2023, that makes it less likely. The Switch is selling very well, and developers have squeezed some impressive games out despite the spec limitations. I have a feeling a mid-gen Switch update would be more akin to a PS4 to PS4 Pro leap, not an Xbox One to Xbox One X leap.

I`m almost sure it is.
PS4Pro have been 20% of PS sales since launch, so if we say that since release PS4 sold 70M that means 16M PS4Pro which is more than several systems done in total. Also this gen will have 7 years duration even when the release consoles were smaller upgrades from gen 7. PS4 SW sale is near passing PS2 with a lot less time and HW sold. So all the aspects we can look at show it likely been sucessful. And considering X1X sold even more relative to base console and gave braging rights It probably is considered sucessful.

I don't think either did as well as they thought they would. Xbox One X especially. It was deeply discounted in the UK within a year and discontinued in just over two and half years.

I doubt we will see a mid-gen console from neither Sony or Microsoft this upcoming generation.

I guess Sony didn`t put a 4k BD player on the console to first not canibalize their standalone player, second because there aren`t much 4k BDs in several markets and third well hardcopy movie have shrinked much more than console. So yes it is good that it is even on X1S, but Sony probably saw it would only ad costs but no revenue to them.

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.