Supposing there is no national dex.

Would that REALLY be a terrible thing?

At least 85% of the total pokemon are entirely useless and pointless outside of filling the dex anyway. The rest are in the regional dex and/or competitive staples.
Filling out the national dex is just pointless busywork.
Keeping it to just the regional dex means these are the first games since the original Red/Blue where it's possible to completely fill up the dex using just the two base games, and that's not nothing