When talking specifically about current-gen for instance, Sony knows a majority of it's fanbase is on the PS4. They'll have those games by 3rd party developers that will cater to both PS4/5 platforms for the next two years or so assuming. Xbox will do their thing with the "not leaving players behind " campaign by allowing their games to work via smart delivery. My assumption is that it's geared more towards to their 1st party content. They've done well to have their backwards compatibility work across their platforms. They have their thing and Sony will have theirs. That's going to be the beauty for those who can afford both next-gen Consoles and a reason why I want both personally. There won't be any FOMO from that perspective.
For Sony, their main focus for new, exclusive content will be on the PS5 only. This was part of Jim Ryan's message months back to where they want people to move to the PS5 as quickly as possible. We would think backwards compatibility would be the driving factor to get the PS5, but it seems now tjhat it would only be for those top 100 PS4 games or so that was discussed. Sadly, I think the PS1-3 games may not make it on next-gen. I've made my peace with that.