It is pretty normal that a certain percentage of pairs don’t survive into seniors. Some may realise that they have no chance because they don’t have the technical difficulty that is needed (Borisova/Sopot), some split up because one of the skaters is injured (Atakhanova/Spiridonov), some ladies get too tall for their partners (Kostiukovich/Ialin)…
I think it is too early to make any forecast about Akhanteva/Kolesov and Kostiukovich/Briukhanov. How can you say that they didn’t survive into seniors when they haven’t aged out yet? OK, they will probably not qualify to senior nationals this year, but that’s not a big deal. They may do extremely well at the junior nationals, get send to junior worlds, medal… and get to senior reserve team thanks to that. Or do extremely well at the Russian cup final and the federation may decide to put them on senior reserve. And even if they didn’t, that doesn’t mean that these two pairs did not ‘survive to seniors’. They may survive, they may just not be among the top teams now, but may come back next year, excel at the Russian cups and be back on track.