I think the choice Sasha made to leave was her own, and there's nothing wrong with it. And there's also nothing wrong with wanting to be the top girl who comes first. I'd imagine all the skaters want this. I just don't think it is realistic for the female Russian skaters. She's not going to be the only girl in Plushenko's rink, unless he has a contract where he trains her alone. And maybe she will be the best there for the next decade. But there are other skaters, and it's possible that one, or many, will become better than her. If they're training in the same team, they'd deserve equal attention as well.
Even if Plushenko doesn't take a direct competitor, but trains other seemingly untalented skaters, what if they improve enough to become a competitor - is he going to hold them back for Sasha, or drop them? When Alina joined Sambo she wasn't anybody's competitor and no one would have dreamt that she would ever be competition for Evgenia. But she improved. When Alena joined Sambo, she also wasn't even considered for the first round of JGP spots. But she also improved and both she and Sasha overtook Anna who was the anticipated new star that year (though Anna was injured).
How many coaches are there in Russia? If they all only focused on one skater, where do all the thousands of hopefuls go? A coach should divide their attention equally among their students. And I don't know any coach who has only one pupil. Liza being Mishin's most successful skater comes to mind, but even then I'm sure Sofia got equal attention, as did Fedichkina when he had her. They didn't catch up, but that doesn't mean others can't.