PlayStation exclusives have a massive fanbase these days and are definitely selling PlayStation consoles. Sony's exclusives on PS4 were particularly appealing to the masses and they routinely made records in sales. Sony games on PS2 barely sold 5 million if they were not Gran Turismo. On PS3 Uncharted and The Last of Us as well as LBP and God of War 3 sold over 5 million while Gran Turismo declined in sales and since 6 hasn't sold 10 million like it used to but overall Sony exclusives became more popular.
PlayStation this gen has made amazing strides in making its IPs more mainstream. They have multiple 10 million sellers this gen, Uncharted 4 is already above 10 million sold, God of War, Horizon Zero Dawn and Spiderman will follow and The Last of Us Part 2 is likely to do so. They have got good public relations except for backwards compatibility which will could be leveraged as a seller for PS5. People already own these games in the tens of millions. Free 4K 60 fps patch with high resolution textures and assets will make them see PS5 as a better value for money and they will buy it. PS5 doesn't just need backwards compatibility but should be the best place to play exclusives on PS4.
and what incentive do developers have to go back to old games and create new 4k assets for them?
that's a ridiculous, unreasonable request (demand?) to have.
let them focus on new games