Unfortunately this is just a thing with 5e: Dex is just a better stat than Str. Dex:
- governs an important (and most common?) ST
- can be used in many places instead of strength (e.g., resisting grapple and shove attempts)
- affects more skills than strength
- adds to your AC and initiative
- affects both ranged and melee weapon attacks and damage
- *cannot* be used for the strongest melee weapons - this is basically the only weakness
(I like Pathfinder's default rule that strength is always added to weapon damage rolls unless you acquire a feat allowing you to add dex to damage. And ranged weapons get no bonus to damage, unless they're composite in which case you add strength. These rules help make dex less OP compared to strength.)
Wyll needs Dex for AC; Warlocks only get access to light armor. If he actually ever lives up to his title, theoretically you want him in or near melee.
Shadowheart's change from Dex=15 to 14 doesn't actually affect much in game. Both give a +2 dex bonus. This change only affects ASIs/feat choice, as now she'd need to dedicate 2 points to increasing it to 16 instead of 1. But honestly, as a cleric she'd probably want to focus on Wisdom.
