My experience reflects the general findings. STR and INT are about equally dumped in 5E with CHA distantly behind. STR started getting dumped as soon as it could be, mechanically, so all the way back in 3E with Finesse and so on. Usually one PC in the party will have max STR and they can resolve anything which actually needs STR - but 5E, like 3.XE, ends up in a place where magic of various kinds can eventually outmode that too. It takes a while though.
I think looking at Saving Throws and some skills is probably not a "fix" for INT and STR dumping.
Illusion effects are rare in D&D and that's not going to change. Moving ones which aren't INT to INT won't convince anyone to not dump INT. Making all secret doors etc. require Investigation likewise won't cause people to not dump INT, because generally speaking, that stuff isn't "life or death" the way detecting ambushes and the like is - as long as Perception is the skill for detecting life-and-death stuff, WIS will be favoured over INT when picking which to dump. Especially as it's the same with saves - the things WIS saves against generally are far more critical to deal with than INT. You'd have to make a really wholesale revision to fix that.
The real reason STR and INT are dumped so much, though, has nothing to do with skills or saves.
It has to do with one thing:
Primary stats for classes in 5E.
Generally it's trivial to get into a position where you can replace STR with DEX for melee. This is mostly a good thing, because an awful lot of fantasy archetypes of melee guys are primarily agile, maybe even the majority at this point. And WotC, for whatever reason, decided most casters should use CHA, even though Warlocks, for example, could perfectly well use INT (also sacrilege maybe but Clerics could honestly use INT and Sorcerers WIS - killing some sacred cows there).
If more classes actually required STR or INT, or benefited from them in ways that were "clutch" (ahem) then they'd get dumped less. Ideally you want to find a way to do this without reverting to a situation where it's just CHA and WIS getting dumped instead, like it was in 1E-3E. Giving bonus proficiencies (whether skill, tool, or weapon/armour) to people with higher INT and/or bonus languages might quickly make it less-dumped (but likely the dumping would just move to CHA).
Also, can we just delete CON as a stat? It's ridiculous. It doesn't factor into anything interesting, it's not a primary stat for any class, but because of the huge amount it boosts HP, it's basically the secondary or tertiary stat for every single class in the game. It's evolved into this stat which is clearly "not like the others". This is only made worse by the fact that it's used for concentration checks and a lot of important saves.
