Start of a new gen is an ideal time, by now PS4 owners have a decent catalog and friends list (etc). I can't imagine anyone switching when you see how great Horizon looks on PS4 you have to wonder what Scorpio can offer to make it worth it. Then, of course, we have PS5 looming round the corner so why not just wait a couple years for true next gen?
Well in line with my last comment, once again, I guess all of this leads back to Game Pass. MS has been making huge strides to reduce barrier to entry on their console. By E3, if we imagine every XBO title on Game Pass is announced to be 4K, EA Access game are all 4K, and then more 4K games coming. Someone can buy into scorpio pay $15 and have access to 130+ 4K games from the get go. With the additional benefit of now having access to Xbox exclusives as well as ensuring the best looking 3P games go forward.
Normally, I would agree with the idea of building into an ecosystem, but this really reduces that barrier and opens up an opportunity for people to catch up quickly (for being away from the platform for 3 years).
And so, I do believe that with proper marketing of game pass and Scorpio, and the right price point, I think you will see people (hardcore market) grabbing Scorpio possibly as a secondary console and just run with game pass or something (more price sensitive will stick with XBO). At the end of the day, MS has provided a great number of options to consumers that have never been available in the past, and so I'm not sure if the standard rules may apply as well as they once did.
I'm not saying that everyone is going to jump on board and go crazy, but if the game pass games are great, and they are scorpio patched, I could see some folks wanting to pull the trigger. The game pass is really effortless when compared to having to wait for sales and what not.
As for your final point. They could also wait for the next xbox after Scorpio -- buy game pass and have 100+ games at 4K ready to play
... I know they are older, but I mean, I assume a lot of those games will be 1P exclusives.
We all know the 1st year of a new generation launch, aren't really leveraging the power of that new console yet, new techniques need to be devised etc. So that first year may not be as big of deal as it once was. The market is certainly changing, if you're expecting a new console in 3 years ... peoples appetites to upgrade earlier will significantly drop.