Not to add to your woes or anything, but it's also FAR more likely for you to run into fire-resistant creatures at that level than it is for your barbarian friend to run into magic weapon-resistant creatures.
Fire-resistance shouldn't be that big a deal at the endgame since there's a feat tax around it. Fire-immune though, that's the pain.
My calculations also ignored the barbarian having a magical weapon at all. At level 20, what type of weapon is he wielding? A +3 greataxe maybe, with other bonuses? At least I have my +3 -- or wait there's no equivalent weapon for blasters. I wouldn't want to encroach on the poor barbarian's territory!
Also if we're talking endgame, a Dragon Sorcerer needs to cast Scorching Ray as a 5th level spell to beat a high level Agonizing Eldritch Blast in average damage. Then we can add that 24 hour concentration of Hex on EB and the Sorc isn't keeping up, period. And the funny thing is that the Warlock's damage is absolutely fine! I'm not looking to drag Warlock through the mud, just the absolute joke they did here.
It's only a nerf if you can show it did that before. As it never did this before, how can it be a nerf now? That's like complaining that there is a ruling that swords now don't crit on a 15-20...well, if they never critted on a 15-20 before, it's not a nerf to reiterate that they still don't do that. Twin Spell always applied only to spells that target just one target - reiterating that is not a nerf. It's right there in the text, "When you cast a spell that targets only one creature". It never said, "when you choose to target only one creature with a spell" or even "when you target only one creature with a spell". But it clearly said "when you cast a spell that targets only one creature". The limit was always on what the spell text says, not one the choice the caster makes with that spell. Clarifying that for those who had misread the text of the ability isn't itself a nerf.
Oh, I can do that easily:
"When you cast a spell that targets only one creature and doesn't have a range of self..."
I cast Scorching Ray targeting Bob. Bob is only one creature, and Scorching Ray doesn't have a range of self. Therefore, I qualify for the prerequisites.
Oh, but Scorching Ray could target more than one creature, you say! Therefore if it could then no no no!
Then by that logic, I cannot twin Fire Bolt, because Fire Bolt can target a creature OR object. Because it COULD target an object, this logic states that it doesn't qualify. As you say, I'm CHOOSING to cast it on a creature, not on an object, therefore Fire Bolt cannot be twinned because it should work on spells that ONLY target one creature. Same goes for Disintegrate and others.
But that is, of course, absurd.
So no, Twin Spell did NOT always work this way by RAW. The intent was not to work this way according to Crawford, hence the change.