Doubtful. The mid gen refreshes were still a ps4 and an Xbox one. These are supposed to be new, different machines and they need to software for that.
And they would get that software, later. For a March launch it would not be needed. The mid-gen refreshes being called a post and Xbox One doesn't matter. Maybe casuals wouldn't want the PS5 without exclusives, but they wouldn't be needed yet.
Yes, weirdly it worked for Nintendo, so far as they went and made old Wii u games have new life, but that was because hardly anyone played them on the Wii u. Pay us $400+ dollars for these new boxes and only have games that you could have just played on the old ones for 5-6 months isn't going to grab most people. It will for us on here, but we're not the typical market.
It working for Nintendo want weird at all. Hell, it worked for them despite people needing to buy those ports of Wii U games at full price, while this hypothetical March PS5 would upgrade your PS4 games for free. And again, it doesn't have to grab most people. Remember, I'm taking about a scenario where the next Xbox is releasing in November 2020 at the same time as most of the next-gen games, while the PS5 launches in March without true next-gen titles, but with enhanced BC and the prestigious status of the best place to play most games for 6-7 months. Even if that only resonates with 5-10 million people, that means the PS5 walks into the holiday season with a 5-10 million sales lead, with a bunch of people who might have gone for Xbox if they'd released at the same time now invested in the PlayStation ecosystem because they bought the only next-gen console available. It would also let Sony commission some early demos of the upcoming next-gen games, or next-gen exclusive modes for PS4 games (like a Legendary Dark Knight mode for DMC5) that would logically be exclusive to their next-gen platform due to it being the only one out. I'm not seeing any real downside to this - you seem to think that people would somehow be mad that there were no PS5 games for their PS5, but I really, really don't think they would give a shit. Especially not if all PS4 games started being branded as 'PS4+PS5' games with 'plays best on PS5' decals. They'd understand. Hell, they'd be happy there weren't 2 versions you had to choose from. Sony has built a brand that people trust, it's why the PS4 sold in drives with nothing but Killzone, Knack, cross-gen games and no BC.