Flaking on original arrangements if you get a better offer

[SpainToday]

I am being deliberately vague here, for privacy reasons.

Earlier in the year, we booked a festive day-out event with relatives. We all agreed on the date, so DH and I booked and paid. We offered to pay, so that's fine. However over the weekend, the relatives have been invited to an extra event, which overlaps with the original event. We would need to leave the day-out just after lunch, missing half of the things we wanted to see/do, if we were going to get the relatives back in time for event 2.

I've told them we can't make it work logistically, so the ball is in their court. They are still deliberating and I'm really hurt and cross. I think this is really rude - if you have plans, then you stick to them, even if you get a better offer?

Not given the distance and the awkward location

Seriously, it was a distance they were expecting you to drive them so it is not hours away. You can get a taxi from anywhere to anywhere.

But normal people don't.

It would cost an absolute fortune to get a taxi somewhere I would happily drive to a couple of hours away.

A lot of people would get a taxi to an airport that was 1 - 1.5 hours away and OP would have to do this journey both ways.
A taxi would probably be less than the cost of the 2 tickets that OP has purchased for them.

We must mix in very different circles then, as I don't think I've ever heard of anyone ever paying for a taxi to take them on an hour to two hour journey.

People I know would use coach or train for that sort of distance if they didn't have a friend or family member to take them.

I agree.

Have you seen the price of some train tickets! If there are 3 or 4 of you it is often cheaper to get a taxi. The cost of a ticket from where I live to Heathrow Airport by train is a third of the cost of the taxi fare and then I have got to get my way to the station.
We must mix in different circles because I would never expect a friend or family member to give up 3 hours of their day to take me to the airport and back. I have never been asked to take anyone either.

All I am saying is that the OP has laid out quite a bit of money. Surely the ILs should be prepared to spend the same on a taxi if they want to leave early.

I agree. The exception would be for a genuine reason, like health/having to help in a family emergency, so I'd want to be sure there wasn't a reason behind it before being annoyed in this case, so ask them.
But as a general thing, one you commit, that's it. But JOMO/FOMO seem to have become big things. Overheard quite a few people being invited to things, and getting a "not sure/see if something better comes along/will decide on the night" type answer. Hate that, and if someone said it to me (again, exception for genuine circumstances) I'd be inclined to say just do your own thing, I'm not hanging around as if you're a princess.

I think, in fairness, that there's been a bit of social redrawing since COVID which is brutal in some ways but fair enough also.

If people were running around doing things 100% of the time before and now find that 80% of the time suits them better, then they'll have less overall capacity for stuff.

It has coincided, for me, with friends all having kids. I now have a much deeper relationship with fewer people, which is overall more satisfying. My husband still tries to keep up with a few people who we've drifted from, but I kind of wonder, what is the point? If we can't agree a meet up that fits into our lives and we can't just meet casually, then maybe focus on the people who are in our lives?!