You're making a lot of assumptions on what players do with their ASIs. In our groups, 16 is good enough and 18 is great. Unless we roll scores, we rarely see a 20 even by tier 4. Other things are just more important to the players I play with and/or DM.
I'd sum this up as "it looks strong, and isn't mechanically weak". Which is what I was aiming for.
I'll agree, I did try to make it attractive and interesting.
You haven't given a single example of an actual character where it would be an overpoweringly good choice. The most concrete you've gone is an elf wizard bumping dex to 18, which (as noted) seems a strange thing to do before bumping int to 18 or 20.
If the most abusive thing you can do with the feat is to tempt an elf wizard to spend a bunch of time doing physical fitness at the cost of a lower save DC, I think that feat is at a good balance point.
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The only Str+Dex builds, where you'd want a 14+ in both, I know of are, in rough order of how common/high the stats are:
Barbarian, who wants Str to attack (reckless and rage), and Dex+Con for AC.
Paladins/Fighters who use heavy armor and/or two-handed weapons and want Dex for initiative/saving throws.
Melee Clerics
Two-handed-weapon-Ranger (which is a strange build, just go ranged; GWM+PAM is roughly equal to SS+XBE).
I cannot think of another PC who would ever consider buying an ASI to put 1 point in both Strength and Dexterity, and the above aren't all that likely to do it either.
In short: I get why it
looks strong to offer +1 str AND dex, but it is way weaker than it looks. Actual cases where it is strong are a good argument why it is too strong, but "that is better than an ASI" looks mostly like it is falling for the appearance of strength not actual strength. And, in my opinion, encouraging people to have str and dex increased has verisimilitude advantages.
Oh yes: and feel free not to reply if you have nothing to add.