The problem is that currently character progress is poor for an RPG, and unfortunately that won't change much at the higher levels.
Care to explain?
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
No, because realistically we are playing on easy right now with bloated health and lower armor for enemies. That would be just on top of the already edited material.
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.
You are partially right, they are giving advantage out like candy right now. It guarantees you to have that bonus 100% of the time, which yes, makes any other feature that gives that advantage pointless. This is the part you are missing, because they are giving out advantage in such a way deters from any other ability that would stack on that as well. Advantage isn't just a +2 its equal to a +5, this by itself out shines bless and anything else ability wise (especially at level one). In a normal situation you wouldn't be getting that as often and as such could rely on other abilities to grand bonuses (bless).
There was a post a while back where they broke down the % to hit with a d20 along with advantage vs not having advantage. While also making it bluntly obvious there is no reason to use any other bonus to hit with how advantage is handed out.
