why not?
sure we can debate what expected success rate should be at a given level, but why is it not a good base?
Because the DC user is extremely favored over the roller
1) the DC user always adds their Proficiency modifiers
The wizard always ads their Proficiency to their spell DC.
The fighter is unlikely to add their Proficiency modifiers to their Wisdom save.
So the DC user had a +2 to +6 favoritism on the roll from the start.
2) the DC user always adds their best Ability score modifiers
The wizard always adds their Intelligence modifier to their spell DC.
The fighter always to add their Wisdom modifier modifiers to their Wisdom save.
Standard array is 15, 14, 13, 12, 10, 8. Or you can roll 4d6d1. Or you can buy up to 15. Then you add a +1 or +2.
The monster will have their DC based on their best score for a Bonus of +2 to +10.
The wizard always choose a high Intelligence for a starting modifie of +2 or +3. And it will grow to +5 or 6.
The fighter is likely to pick Wisdom as their 3rd or lower best score. So its a +1 and almost never grows.
So its a +1 vs anything from a +2 ro +6. Maybe a +10.
Altogether the DC user has a DC of 12 to 20+. Whereas the roller is rolling anything from -1 to +10.
Oops. Forgot.
3) the DC user always chooses the targeted save.
Not using Con or Fort saves vs big monsters.
So really the DC user has a DC of 12 to 20+. Whereas the roller is rolling anything from -1 to +3.
Yeah buddy.
Thats bad.
Now with abilty score.
The fighters 13 Wisdom is a +3 vs a
Mage Apprentice's Intelligence of 16 so DC 16.
Mages Intelligence of 17 so DC 17.
Archmage's Intelligence of 20 so DC 20.
Now you gotta chance.