We can’t draw arbitrary lines. We either have a monarchy or we don’t.
The royal wedding, well not even of the year, recently took place between Princess Eugenie and some posh bloke (Jack Brooksbank). The event didn’t just get the attention of those who support the monarchy and any excuse to revel in the chance to bring out the British bunting. It also became a massive talking point for those who objected to tax payers having to foot the bill for the security for the event. It was an absurd position.
It would be easier to have sympathies if people had also objected to paying for all of the other royal weddings but the foundation of this argument was essentially that Princess Eugenie was not royal enough. That argument has no relevance though. We either have a monarchy or we do not.
It utterly doesn’t matter how popular someone from the family is because it’s a matter of security. That’s right, they paid for security and not the whole event for a reason. If we have a monarchy (and just to be clear, I’m very much on the side of getting rid of the lot of them), then they have to be protected by the state. It is a responsibility the state has to have. Part of protecting the royal family means protecting the ones most likely to be vulnerable, exploited, have less protection and have less attention. An unpopular member of the royal family can still be a target for an attack or terrorism or kidnapping (but hopefully in a story most unlike Black Mirror).
“It’s a matter of security”
They are a security liability as there is a chance they could be a target. Princess Eugenie may be irrelevant but that hasn’t stopped the press giving her attention constantly and giving her a platform. She may not have active political power but she is absolutely a notable figure in society. We can’t draw the lines around just the figures we like or are more popular. That actively defeats the purpose of a monarchy in the first place, which is to exist beyond whatever the public want or think.
If we don’t want to foot the bill for security for a family that represents nothing more than elitism then the only thing we can do is ditch having a royal family completely.