(spoilers for Diamond Age follow)
Mmm, I understand what you're getting at but from what I remember of the book (it's been a few years, please forgive me) the two you're talking about that failed were relatively privileged little girls who in at least one case had separate tutors, school etc, and this played into why it failed, because they abandoned the TYLIP whenever it challenged them or made them uncomfortable. Moreover, TYLIP didn't just work with Nell, it worked with the whole Mouse Army, hundreds of thousands of young girls. Nell not having her birth mother and being functionally "adopted" by the ractor whose name starts with M was a huge part of Nell's character growth, and a parable about children failed by the system who can be so much more if they have someone to love them, but I don't think it was the only reason TYLIP worked. I do remember the book gesturing at what you're talking about though, specifically when it was examining the difference between Nell and those two other relatively privileged little girls. Moreover, my understanding was not that those other two girls had a purely automated TYLIP, it was that the ractors their TYLIPs automatically hired were short-term gigs, while Nell's was someone who stuck with her long term (and was invested and nurturing, as you say). In fact I'm not 100% sure, but I think the whole Mouse Army may have had fully automated TYLIPs that ended up turning out quite well for them. The general trend seems to be that young girls who had no other parental or authority figures bonded very well to the ~~wire monkey~~ TYLIP and developed well because they stuck with it, while those who had human alternatives chose those alternatives as soon as the TYLIP challenged them and therefore did not stick with it and succeed. Doesn't exactly leave us with a policy prescription, but it's still an interesting story.
Basically, I agree that anyone seeking to replicate the TYLIP should at least have an answer for this objection because I see it raised every time TYLIP comes up, but the book's actual contents on this issue is pretty complicated and I'm not sure it's obvious that the books version of events is accurate to real life, or even that it maps cleanly onto a "human touch good, wire mother bad" framework.