Microsoft doesn't need an Xbox-focused strategy to perform better than with the huge blunders they committed with the Xbox One. The execution of the Xbox One was so awful that even a mediocre execution of Scarlett is bound to lead to gains gen over gen.
You overestimate brand loyalty a great deal. History has shown time and time again that the majority of gamers has no problem to leave games, save data and achievements behind, because new consoles are bought to play new games. As for starting from scratch, I clearly refered to installed base and there's no debate that the PS5 will start with sales of 0.
I disagree a bit. While I would not overestimate brand loyalty, history also has shown that it exists. Look no further than PS4. At launch the library had a lot of 'definitive ports'. But people bought the system and rebought the games. A friend literally said, he rebought something (don't remember, might have been Tomb Raider) so that he has something to play with his new PS4, while waiting for more games. A lot of PS4 sold on brand loyalty. And the promise of a great library down the road was fulfilled, so I don't see thta brand loyalty destroyed.
But even more counts the network effects Xxain talks about. Network effects are really strong. That is the reason competing with Twitter and Facebook is so hard, people don't want to lose their friends, followers, likes and so on. Same with the friendlist, achievements and more on consoles. And just think about it: you own a PS4 and have PS+. As PS5 releases you might not be interested, but each month you automatically get free games for PS5. The deal looks sweeter as time goes by. Cross-buy means you don't have to rebuy all the smaller download-games you purchased and are available on the new system. And so on. Sony has a lot of this stuff to pull you over to the new system if you are already invested in the previous one.
Nintendo by the way sucks at these effects. Going from Wii to WiiU and 3DS you had to readd your friends. Switch at least offers to send your previous friends an invitation, so it is a bit better. You had always to rebuy all you digital downloads on each Nintendo system, even as the same was available. This was true for virtual console, although I remember Nintendo offered a discount for VC-titles you owned on Wii. Now VC is dropped completely. Miiverse was just canned. Just think about it, if Switch had retained Miiverse, all the WiiU and 3Ds owners would have seen posts and screenshots from games on Switch in their Miiverse. This could have been a pull-factor. So yes, you are right in regards to Nintendo. For Nintendo each new gen starts at zero. But this is a failing of Nintendo, others do that better.