I think not allowing it at all is too extreme. There are plenty of households with older cats that often adjust to a kitten and even take on a parental role. Also, there are ways to enrich a kitten's life even if it's an only child -- you just have to be able to spend plenty of time with them and provide a ton of loving, teaching, and toys. My kittens' siblings were adopted out alone, and they're still well-adjusted.Yes, you made me see a risk of scaring people off. But at the same time they need to be aware of the consequences to manage them, and while “traumatized” is a big word, as you said-young kittens with no mom do need special treatment at least not to bite/scratch. And in your case, you had two kittens. When I see a post of someone adopting 8weeks old cat child alone it simply breaks my heart. And frankly I don’t think it should be allowed even in the extreme circumstances (some European shelters will not adopt out one kitten at all, and I also saw that the shelter in Seattle has the same rule)
I do think that potential kitten parents need to be better educated about young kittens' needs.
One last thought: many people are reluctant to adopt two kittens at once (even though it's often easier for you and them, which I've pointed out elsewhere on this site), so I fear that more kittens would end up in shelters for extended periods if there was absolutely no possibility for them to be adopted out alone.