Well, it makes casting a Lightning Bolt or Fireball against dragons a no-brainer! The best DEX a chromatic dragon has is 14 for Black dragons. Even at lower levels with a modest +6 spellcasting modifier you will be successful 65% of the time, but in the current system with the DEX save, the Black Young for instance has a +5 modifier because it is proficient and would need a 9 or higher against your DC 14, reducing your chance of "success" to 40%. Since 3 out of the 5 chromatics have DEX 10, you would only need a 4 to hit them with your spell or an 85% chance! But they all have save proficiency, even the worst (White with +3) would have a 50/50 chance of saving.
Basically, you are making spells much more effective since not as many targets have a high ability score without proficiency in the save as well. I've attached the spreadsheet if you want to look into it. Maybe there are other considerations you have that I am not seeing, but unless someone wants magic much more effective in their games, I would be very hesitant to implement such a system.
(I know you said you aren't using this, just your offered idea, so please understand the "you" is generic and plural.
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