I think there are two different categories of persons we are talking about in this discussion:
1) Junior princes who get an earmarked sum in the Royal Household budget such as Prince Joachim of Denmark, or Princess Astrid and Prince Laurent of Belgium.
2) Junior princes who do not receive a specific set amount from the State, but are reimbursed from the Royal Household lump grant when they perform official duties on behalf of the monarch. That is the case, as far as I understand, of Prince Carl Philip of Sweden (and previously his younger sister, Princess Madeleine), and I believe is also the case now of the Princess Royal, the Earl of Wessex and, previously, the Duke of York in the UK.
Whereas #1 is being phased out in most monarchies, I don't see #2 going away anytime soon. In the UK in particular, I don't see a scenario in the near future where siblings of the monarch will not be at least part-time working royals, and I don't see why Charlotte and Louis should be different when George is King.
Personally I am not a fan in particular of the Spanish model where the children of the King (e.g. King JC's daughters) are officially members of the Royal Family (and undertake official duties) while their father is on the throne, but cease to be so when their brother, the Prince of Asturias, becomes King himself. It is unfair on the Infantas, who are caught up between having a public role and building a private career, just to be ditched from official public duties later, and it is also a heavy burden on a young royal couple with underage children like Felipe and Letizia, who all of sudden have to shoulder pratically all royal duty alone, relying possibly only on aging parents/ parents-in-law.