I guess this makes sense but presumably the PS5 version would have better re-sale. The other downside is the system is so fucking stupid that it will start to install the PS4 version every single time you boot up the system with the disc in. This still hasn't been patched for Tsushima ~6 months later.
If buying it digitally, you may need to download it and run it to upgrade like the Director's Cut of Tsushima..if that's the case it wouldn't be worth the time imo. If it just gets you the PS5 one as well automatically, then it sounds like a good idea.