That's is not the "history". Several big name medalists, Shelepen, Lipnitskay, and few others, switched from Tutberidze to other coaches, and it was well covered and discussed. Med did not set any precedents.
Polina's "decisions" started much earlier, she wanted to quit skating all together in 2016 and then in 2017, and said so herself. Her parents insisted that she keeps trying (since so much was invested). Her parents also were AGAINST her leaving Tutberidze, but Polina wanted a less competitive atmosphere. All this was said by Polina herself. It is absolutely BEST for Polina to practice in a more calm, less competitive environment. People are different.
That is your opinion of Alina's personality.... But her actions tell otherwise. She is acting very smart in public and with media, concentrating on her work, shows and endorsements, and not any more "unstable" than any other girls at this time of the season. She is doing her job and not getting involved with "external BS and game playing" unlike others.
There are arguments to be made (and have been made) if Med's reaction and decision after the Olympics are level headed. Her own mother was very much against moving to Orser/Canada, and only agreed to ditch her career in Moscow and accompany Med "on trial basis" and because Med strongly insisted. Med would not have switched if she won the Olympics. She was driven to switch mostly by her ego.
Anyone "may move", or "discover a cure for cancer", or join a monastery..
Kostornaya, or any top-level skater, still ranks where he/she ranks in a given season. Kostornaya (since it is your example) is #2 Jr. behind Trusova right now internationally and nationally. If she stays with Eteri, or moves to another coach, she will still be currently #2, unless she learns 3A and/or quads, or if Trusova starts falling in competition. When she moves to Sr. next year, she will still rank as she would rank with any coach, and where she ranks as Sr. will be clearer at the end of this season, when we see how other Sr. girls do. Being #1 at your rink maybe a mental pleasure, but in competition one still ranks where one ranks based on performance and abilities.
Eteri certainly brought out the best in Kostornaya artistically, so the only reason to switch is nothing else but an EGO and wanting to be the "queen of the rink" for sake of an Ego... and that (ego) should be applied in moderation, to the "athletic parts" and not "social issues".... Sometimes "bigger and stronger ego" means sticking around in a tougher environment when the stakes are high.
Kostornaya is a fighter, and is VERY competitive, and not just in skating... Plus..... there are no top coaches left in Russia who do not already have their own #1 Ladies, plus few more in #2 and #3 spots. This season will be Trussy with Quads vs. Kostorka with magic flow...

. Next year is next year... hopefully they are healthy and alive.