This would be a relatively elegant solution for difficulty options that accounts for skill checks, attack rolls, and spell save DCs (something that high ground/backstab doesn't).
Story: Proficiency +6
Easy: Proficiency +4
Normal: +2
Hard: no bonus
Very Hard: -2
Adjusting proficiencies or any numbers period related to skill checks or attacks will deter from bonuses gained in other areas. Spells, abilities, racials, inspiration points, ect.
This is why Height advantage and backstab is a problem in the first place, it takes away from already setup features from 5e. Increasing proficiency would just cause another problem.
I'd argue that the larger problem of height advantage and backstab is that it grants Advantage, which means that all other sources of granting Advantage (spells, class abilities, shoving prone) are invalidated. And because Advantage is so powerful, these options outshine ~all other non-tactical options in combat.
A higher proficiency bonus, however, stacks with all other bonuses and
doesn't limit tactical options. With small +2 increments dependent on difficulty, on most difficulties it will still be useful to Bless/get Advantage.
Normal difficulty 70% to hit -> Advantage becomes 91% (+21%)
Easy difficulty 80% to hit -> Advantage becomes 96% (+16% = still pretty powerful)
I would likely still make use of Bless/Faerie Fire/etc if I was playing on normal or easy.
This option breaks down a bit when you consider that skills you don't have proficiency in aren't affected: my character not proficient in persuasion will have the exact same bonus in Story and Very Hard. But this is a much smaller issue than either highground/backstab Advantage or adjusting enemy AC & HP to change difficulty.
