How often are extended competitions asked for though for most checks? I would submit rarely.
A portcullis is blocking a path... the DM asks for a Strength check. The "buff superman" with the 20 STR rolls a 4 for a total of 9... and the STR 5 "weakling" rolls the 19 and gets a 16, thereby raising the gate because the DM doesn't ask for an extended check in that situation. Thus, that tells me that it's not the ability score that is determining how strong these two PCs are... it's the d20 roll. The d20 roll is what gave that one PC the power to lift it and didn't do the same for the other. Even with the supposed bonus of the supposedly "buff superman's" high strength score.
we get a lot of group checks to describe some extended activity.
IE: stealth, 6man party, we need 5 successful stealth checks, every stealth that beats DC by 5 or more adds extra success.
with DC 15, party with higher DEX will get more successful encounters than a lower DEX party.
But I'll be honest here... my quibble isn't the numbers themselves. It isn't the fact that it's the die roll that really shows us how much a PC is on a thing. But rather my quibble are that people try and define and describe their characters based on their ability scores. Because those descriptions just aren't right. The fact is, you aren't an "idiot" with a very low Intelligence score, you aren't a "clumsy oaf" with a very low Dexterity score. Rather, you are merely just less of those things than someone else might be. Someone else will usually be more charismatic than you... but it doesn't mean you are therefore "repellent" with your 6 CHA. Because once out of every 5 rolls you're still hitting that DC 15 check.
will have to run the numbers again with 3d6 variant instead of d20 for skills.
now about describing, yeah, it's not perfect but it is good enough of a descriptor.
and you are comparing yourself to best in field as an adventurer.
with 8 DEX you are not really noticeable clumsier comparing to average Joe, but next to DEEX 20 rogue? yes you are.
and 8 CON is perfectly fine for your village, sure you might get bedridden for couple more days a year than someone with 14 CON, but in the end, it will matters very little, but for adventuring? that is very problematic. especially on levels 5+
and it gives headache to DMs for making encounters.
that is why in 5E(and earlier) with pointbuy we see CON as the "14 stat", little investment, but nothing special.
you need con, but you dont need too much CON, you need to be within your group.
and I have seen that in this 12 years of 5E, 90% of characters made is with 14 CON,
9% are 12-16 and 1% is everything else.
because here 8 vs 20 is a HUGE difference.
5th level wizard with 20 con is a tank, while 8 CON is a one-shot target
it's 47 vs 17 HP.
"average" fighter with 14 CON has 44HP, a barbarian has 50.
with 14 CON, that is 32HP. with then 12 or 16 CON, that moves is to 27 or 37 HP. Much more manageable spread of HP.