It's very difficult isn't it, when the more affluent families with children who've already had lots of experiences say 'we won't be sending our kids because they've done those activities before' - they might add that they did them more cheaply or free with another organisation or with the family. And then there will be families who say 'we wont be sending our children because it's too expensive/we don't think we should have to pay for school for school stuff' - so basically, lots of families are not supportive for various reasons.
In tHe end, each family obviously has to weigh up what to spend their money on, what to opt into and what to opt out of and make decisions based on overall finances, personal values and bigger family spending picture. I think it's a shame if children miss out on what might be the only residential trip if Primary school at the end of Yr6 - very much about time away with the year group as much as the activities themselves, although again, those adventurous activity centres which many schools use do provide lots of opportunities for personal growth, even to children who've done them before.
Secondary is often a bit different. Most decent sized schools offer a pretty wide range of residentials and the idea isn't that people go on all of them. Most will be in holidays and those v expensive ones are usually only for a group of up to 50 - people who don't go are certainly not going to be in the minority. What is a bit more tricky is trips related to the curriculum or during a school 'activities week' so during the school day. Curriculum trips need to be offered to all regardless of ability to pay, but if many cant or won't pay, these tend to be cut or reduced as schools simply can't subsidise many places or expect other parents to do so. However, if the trip is offered, all children should be able to go.
With activities weeks trips, I've seen good models where several day out/short residentials/school based activities were offered and families chose a selection for their child, based on their preferences and ability/willingness to pay - free things were always available, often in school. Most children spend some time in school and some outside and nothing is for the whole year, so there is less of a sense of exclusion. What is more difficult is the whole year residential in activities week which has to be paid for or a few get left behind. This model seems to be the worse if all - no funds to cover those who can't pay, plus no requirement to provide trip for free if it's not curiculum based, plus most people going and only a handful being obviously left behind with inferior activities.....fortunately this doesn't seem to happen much and schools work hard to avoid the scenario.
I'd say, that if school is running one trip which is for everyone and plenty of notice is given, then even if you've done the activity before it's worth going and deciding to value the 'being part of it with everyone' aspect. If it's an optional trip amongst many or one where only a few ever go, then either go or do t go - it won't matter really. It's distinguishing between the 2 types which is important I think and a shame with the first type when parents choose not to send their kids.