Dont count on it.
MS has made it clear that Stadia will be their major enemy and that is because Stadia is doing what MS has wanted to do all along. Those studio's MS acquired were not to bloster strength against Sony, but to ensure enough content for the fight for "games as a service" war. MS could care less about winning the console wars and the only reason they're going with another one is just to keep fans on board to help win that fight.
I definitely think you are underestimating Sony. They will not start from scratch. Sony is making moves that ensure all those useres who loved PS4 or PS in general will have hard time not choosing PS5. PS5 is looking to be very similar to Wii U and Switch where Switch literally ate the Wii U and just built on top of it but in PS5's case its not just tech, but user stats: trophies, games, save data, , PS store, Ps now ext. If you gamed heavily on PS4 why would you not choose PS5? You start where you left off.
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.