It seems Sony has updated a patent they have posted in March which might hint at backwards compatibility for the PlayStation 5.
Here's a summary for the patent named "Remastering by emulation":
Each asset such as a texture called for by legacy software such as a legacy computer game software has a unique identifier associated with it. The unique identifier can be rendered by imposing a hash on the asset, and then the asset stored with its identifier in a data structure. An artist remasters the textures for presentation on a higher resolution display than envisioned in the original software, and stores them back in the data structure with their identifiers. The original software is then played on the higher resolution display, with asset (such as texture) calls being intercepted, identified, and the data structure entered to retrieve the remastered asset having a matching identifier. The remastered asset is then inserted on the fly into the game presentation.
Personally, I don't think this kind of emulation will be similar to the kind performed on the actuality. It might be a new breed of emulation, but that's just me.