How would they explain it? They removed BC on PS3 with 0 backlash, didn't put BC on PS4 and had no hurdle, MS put BC on X1 and it didn't even move the needle.
You'll have to justify the extra profit Sony would do with BC or how much you can accertain it would lose without BC. Because all I see is you and others wanting they to put a feature for free that would remove revenue from double dipping.
Doesn't seem something as certain as you like to think.
We can't tangibly say how much it adds to a platforms appeal and there has never been a clean case study for us to use to determine whether it does helps sale a lot during launch periods. PS3 was $600 and had a price cut to coincide with the removal of BC, the Xbox One didn't receive BC until 3 years in. What we can say however is that Console's for which BC makes sense have always had it.
Unless PS4 BC adds some notable cost to the PS5, history suggests that it will have it.
Also I think some of you greatly overestimate the value of remasters for Platform holders like Microsoft/Nintendo/Sony, these are not primarily software companies. Many people who bought The Last of Us on PS4 had never played it on PS3, remaster sales of recent games will always give slightly misleading impressions since said games would have just continued to sell in their OG form (of course not necessarily as good). In the race to establish a 100m+ platform which sells equal amount of games each year, I don't think think they're gonna give up a potential edge over the competitor at launch just so they can sell 10m remasters across the following years.
We've seen with the PS3 and Xbox One how crucial it is not to mess a consoles launch. And if PS5 and Xbox Next are launching within 12months of each other and one of them has native BC and the other doesn't, that will be a huge talking point and probably one of the key distinguishing factors between them. Again there has never been a fair case to compare in the past.