You can't compare both machines like that because your premise is most probably wrong. From the leaks we have Scorpio is Xbox Two. Microsoft won't mandate forward compatibility to third party developers. That won't happen, both machines are too different. New CPU, new memory architecture, new GPU tech etc.I don't understand why anyone would want them to ditch forwards compatibility and start clean with PS5. I just don't see the value in doing that. People have this nebulous idea that they'll be able to do "magical" things if they have a clear cut generation. I just don't see it. Look at DX12 and now the Vulkan api. They are high-performance and highly programmable, and work across a broad range of hardware. Sony should make a slightly higher-level api (or use GNMX if it's suitable), enforce it as the default for PS4 Neo code-path and then use that as a transition for PS5.
If PS5 comes out and PS4 is effectively "put on notice" with limited support, then they'll have a $400 PS5 and a cheap PS4 Neo (probably at around $250). If it works like last gen, you can buy that cheap PS4 Neo but it will have a limited and half-assed library of new games once PS5 ships. Each year it would get worse and worse, effectively making PS4 Neo a platform for old games. If at the same time, Microsoft offers forwards/backwards compatibility and doesn't have a clear-cut break in generations, they can have an Xbox Two for $400 and an Xbox Scorpio for $250. Their story could be, "Go ahead and buy the Xbox Scorpio now, as it will play all of the new games for another three to four years, and you can upgrade to Xbox Two when the price is in your range." There's no way that Sony's option would be more consumer friendly.
Sony needs to kill off the idea of generations and just go ahead with overlapping products.
I don't care what Spencer is vaguely saying. It's only twisted PR and Rise of the Tomb Raider taught us that he is the master at twisting words and meanings. The thing is not feasible. Too much work for devs. The best you can hope is that Microsoft games will run on both machines... at least the first years.
From the 4.2tflops rumored specs Sony is the only one that can realistically mandate forward compatibility to third party developers (including indies) during the whole Neo lifespan.
Again Scorpio to XB One is what X360 was to XboxOG. XboxOG relative failure (4 years) -> X360 global success (8 years) -> XB1 relative failure (4 years) -> Scorpio?