Yes INT would be popular, just like WIS would be popular with builds that multiclass with WM. Just like there is still benefit with using CHA. It should be up to the players to decide how they want to build because this game was built on builds and player choice. If you don't like something or don't want to take an ability you don't have to take it. You are not being forced to take it. If it should be up to another class feature than WM then which other class feature would it be?
If it's so wrong so unreasonable to have pure Wild Mages potentially use the highest of your INT/WIS/CHA for spell casting DC then either delete the entire ability all together or implement a restriction that you must have atleast 1 level in a different spell casting class to access it.
"Mixed Magics: You may now use the highest of your INT/WIS/CHA for DCs of your spells. Your Caster Level with spells is now equal to your Character Level instead of individual class levels (max 20). This does not change the caster level of Rune Arms, Wands, Scrolls, or other Items. Must have taken at least 1 level in another spell casting class."
The difference comes from the design direction. You are right as you're inclined to allow various possibilities to people, while I am also inclined to that but not permissive. Player choice, yes, and that only comes when the game allows. You don't take an ability that you don't like but as long as the game allows it. I like the player's freedom of choice as well, but I am not in favour of merging everything into a single solution, I've been saying. This is why I don't like FvS using both of WIS/CHA in spellcasting ability and SP pool (but It's understandable since they were designed to be greedy. Sin of SSG.) But they are not that excessive.
"Must have taken at least 1 level in another spell casting class." might look fine, but it's also weird to me as you can be INT/WIS based spellcaster with only CHA classes, but also I don't like that restriction exists. I feel this is closer to restricting the player's choice.
- [Mine] "You can take it if you like it, but it's only effective when your main stat is CHA."
- [Yours] "You can take it only when you do multiclass with other classes."
It's tricky, isn't it? I wouldn't say every person would feel the same, but for me, I feel my suggestion doesn't have restriction than your suggestion even if mine doesn't open more possibilities than yours. "There is no option existing." over "There is an option but you should do something."
Now, let's talk about "other class feature". Why shouldn't this be outside of WM exclusive? Why can't wizard have this too? Archmage tree having the identical enhancement but as INT alt. Cleric domain "Magic" to alter WIS alt.
If you have read my posts, I am more interested to make multiclassing caster classes more common by allowing caster levels shared without this enhancement, but DC check is another issue.
Note that I also don't like modern designs that every ED spells being INT/WIS/CHA the same, but ED is difficult to design and I can understand it. There should have been something altering spellcasting ability in the game like my suggestion.