I understand the differences were due to technical limitations but on modern hardware that can compensate for those limitations, it shouldn't be an issue to iron things like frame rate, high res textures, or even change the texture used for water to mimic the water effect used on the saturn version while still maintaining the better smoke effects from the psx version.
I'll even go so far as to wager that for pc, if the game doesn't match visually what is seen in the saturn version, that those assets will be modded in before too long.
You are correct that today, the devs could fix all of the PS1 - Saturn differences and give us the BEST version. However, if you have played both versions you would just KNOW that this HD version is based on the PS1. They did not use (or could not use) Saturn's original-better version of backgrounds, shadows. etc.
Your post is technically correct, modern hardware does not have the same constraints, but the engine they seem to have used as the baseline is the PS1 one and they did NOT update the things that looked-worked better in the Saturn.
1997 constraints are not the same thing as 2019 dev team scope. Their scope seem to be Grandia in HD, and the added scope of "make it the best version by manually fixing all that was missing/different from the Saturn" was not included.
Sample of scope:
- Grandia HD - Check
- Grandia with filtered textures - Check
- Better Parm town ground textures and baked shadows - NO
- Higher res 2 D backgrounds in battle mode - NO