All sorts of spells threaten everyone, and folk with d6 hit die especially, be they Sorcerer or Wizard. But even if Feeblemind was the only thing that could utterly wreck a Sorcerer, it is still a one shot defeat that any Bard, Druid, or Wizard built for level 20 dueling should be throwing at them.
As for countering it, someone not being able to counter your Subtle Counterspell only matters if your Subtle Counterspell works in the first place. The odds of a 20 Charisma Sorcerer even successfully counterspelling an 8th level spell without using their own 8th or 9th level slots is only 40%, and you can't Counterspell it if it is cast from between 61 and 150 feet away.
I'm not saying Sorcerer isn't a strong option, I'm just saying that a Sorcerer's whole strategy against any class that can cast

they are sure the enemy has used up the two appropriate spell slots.
Basically no, wizard spells can trully threaten a guy that heals 210 hp in a single turn that has 200 hp. Well It's Divine Soul Sorcerer.
Utterly wreck a Sorcerer. OK?
Counter
1) Subtle Counterspell is the simpliest, but depends of range.
2) Resilient feat (Inteligente) + Headband of Intelect (19 Inteligente, uncommon) + Favored of the Gods (+2d4 save divine soul, average +5) + Ring of Protection (+1 save rare) + Stone of lucky (+1 save, uncommon) .
It's +17 inteligence saving throw against DC 18
Even with 1 on dice, The Sorcerer wins.
Remember portent is 3d20, average dice is 5, 10 and 15. Expending the lowest dice to force the sorcerer to lose initiative. That is not yet guaranteed, because the wizard can still roll less than 5 on dice.
. So, It's 10 and 15 portent dices. Well...
3) Staff of charming, It completely counter feeblemind combining it with the counter 2. The feeblemind spell returns against the original caster. Good lucky.
4) Long range battle, the Sorcerer hits at 620 feet x 150ft wizard. Well, we know what happens here.
5) You must see, both portent and feeblemind spell require
see the target.
But the sorcerer can stay
16h on extended etherealness spell, allowing him to set your position whatever he wants, back up to counter 1 and 4
There is
100% chance to defeat the Wizard.