Ideally, children would be left to take on as much responsibility as they are capable of accepting. In reality, they are likely to be given as much responsibility as they are actively willing to take on, which means provision has to be put in place for those who cannot or will not take responsibility, or who need protecting from others' irresponsibility!
Not all parents can be bothered or afford to get their children child-sized suitcases. Do you propose those children be required to carry an adult-sized suitcase, packed full of heavy and unnecessary items, up lots of stairs? For those with sensible suitcases and backpacks, I doubt they will have these wrestled off them if they wish to carry them by themselves.
As for unpacking their own things - what's wrong with living out of the suitcase? Did they actually say anyone would be unpacking anything? I do not blame the school at all for wanting to avoid the inevitable mixing up of children's possessions if they are left to unpack things themselves, without supervision, that cannot be put straight into carefully marked storage compartments so as to avoid getting them mixed up with other children's stuff (and a suitcase is generally a perfectly acceptable marked storage compartment, if you ask me...). What adult has managed to come home from every holiday with all their possessions intact, even when not having to share their accommodation with 20 or 30 8 year olds? If each child has their own room and chest of drawers they could unpack everything. If not, then I can see why the school would balk at the idea of unsupervised unpacking.
I would not trust my children to wash themselves unsupervised at age 8 if there were lots of other 8 year olds being silly, giggling and taking advantage of no-one standing over them. I don't think an 8 year old playing about with their friends is old or mature enough to understand that not brushing their teeth for four days running is bad for their teeth - they need an adult standing in the bathroom with them to remind them of that fact. It may be fine to trust them at home (and be able to check their breath afterwards), but not fine when there are so many other distractions.
If the meals being served are not part of a buffet service, then why would you not leave one of the adults to put a heavy pot of casserole into the middle of the table and serve portions up into one bowl at a time to hand out? Why require them to waste hours getting children to push each other aside in order to get to the serving bowl and serve themselves unfairly small or large portions of food? I'm not aware of most families serving their food up that way. They should be required to clear their own plates away afterwards, though.
Some children at age 8 are STILL not physically capable of wiping their bottoms particularly well. Combine this with not supervising their washing habits and you are asking for a big stink.
Some children at 8 will be spending their whole time on the trip feeling homesick and miserable and will not be able to cope with not only being away from home but also being required to do things they are not used to doing for themselves - or possibly even doing things they are used to doing by themselves but only when they have the comfort of knowing their parents are nearby to help them if necessary.
School trips are not the same thing as brownie camps. All children are supposed to be included on school trips, not just those who want to be brownies.