We should also not forget that Joachim has an excellent judgement of people and he has insider knowledge - he knows what makes Frodo Frodo while we can only speculate what could be wrong. He also knows that Marge, at her present age, will not continue working for a while - so then king Frodo the unable will have to take over who at times has no engagement for weeks, misses the anniversary of the liberation of Auschwitz and goes hunting ... His wife is clueless and ambitious and clearly no fun to work with. And Christian is nearly a child. This is not likely a successful team to represent Denmark on a national or international level.
If he lived in Denmark when that happened - people would expect him to step in - and Marie. Then the comparisons would start - Jokke vs. Frodo, Marie vs. Mary. In the end they would have to hand over the show to Mares and Christian and disappear again. If he would not step in he would be the mean brother. So he decided to disappear while mom is successfully running the show and in future, if needed, turn up at times but also say "Excusez-moi, je suis ..." at others.
Jokke might be seen as arrogant and snooty, but on the positive side, he has done a good job with his children (Nikolai and Felix seem very well adjusted for their position, and Henrik and Athena seem polite and well behaved for their ages) and he is clearly very intelligent and with it, capable of reading people and situations very well.
Jokke knows what is going to happen - basically exactly what you said. Marge does not have a lot of working years left, but his big brother does not have it in him to do the work and duties of the King. When the worst happens, Christian will either still be in high school, or he will only have very recently become an adult, and public opinion will turn very fast on Frodo (and by extension, Mor) if Christian has to quit his studies early or basically go from "high school graduation to full time Royal". That is when narcissist Queen Mary will be begging Jokke and Marie to come and pick up the slack because Frodo is useless and X and Izzy aren't old enough to be full time royals. Once Christian is old enough to take over duties and Isabella is old enough to serve as a "spare who does the less glamorous engagements", it will be time to toss Jokke and Marie to the wolves to make sure the now "main" line is protected.
Because Jokke is smart, he's knows this is how it would play out.
Instead of fighting Derf and Mor, he's basically said "Ok, you win, country and spotlight is all yours".
Mor is probably delighted, thinking she has won. They are gone! It is all about her and her husband and her kids now.
But she hasn't won.
I don't think either Derf or Mor realise that what Jokke has actually done is handed them the entire rope without a fight, knowing that his big brother and his self obsessed sister-in-law are dumb enough to eventually hang themselves with the rope and then when they do, everyone will finally see how lazy, useless and entitled they are.
Jokke is playing a very clever long game here and it is one that allows his wife and his children to have happier lives away from the crazy extended family.
I just feel really sorry for Christian in this whole mess, and Isabella too, if she ends up having to play the same "we'll love you for a while and then toss you to the wolves once your brother gets a wife and produces an heir" spare role Jokke did for years.
He's still just a kid, and it is not his fault he was born to a lazy useless lump of a father who seemingly refuses to deal with his mental health issues so he can be a capable and fit king and parent, and a mother who is a self obsessed narcissist. Isabella in particularly didn't ask to be born to a mother who clearly resents the hell out of her for existing and treats her like shit because she has seemingly unfairly assigned blame to her daughter for the fact that her husband seemed to stopped loving her around the same time Izzy was born. The whole family is a fucking mess that needs a LOT of family therapy. The poor kids are the victims of horrible parenting.