Well, they had different plans at first and then had to scramble to get Kostornaya to the last 2 JGP competitions. Not everything goes according to plan... Kostornaya was also fortunate that several skaters such as Gubanova, Fedichkina had poor showings during JGP. It's not something to rely on. Kostornaya's quite a special case.
When it comes to pressure, the Russian girls tend to compete so much and so heavily from such a young age that these skaters will be able to handle pressure, even though international competitions always are quite different of course. If they couldn't handle pressure, they probably wouldn't be in conversation for JGP spots. And this will likely become even moreso the case in the future as the general skill level of the skaters is consistently increasing. Zagitova herself didn't really go through that entire experience, however. If she had been raised in Moscow with such early age results, she wouldn't even have been invited to competitions.
Still, if we're thinking about next season, I don't think there'll be very many repeats outside of Sinitsyna and Kanysheva. There are so many good new girls.
No, I don't think we'd complain about her not having spots. Or even Chistyakova. This year's age group's so weak, there's not much to spread wealth to. Kostyuk indeed is one, but she can be a bit of a disaster a little too often.