I think it all depends on the scale of parties, for larger parties at other venues, disposible stuff makes it easier to tidy up, but you could for example all club together to buy some of ikea's 90p plates and cups, buy enough between you for a party of 30, and whoever is having a party gets to borrow them.
Party bags are a tricky issue, as children expect them now, and I have even seen tantrum from 4yr old when given book, sweets and cake, but no 'bag'. It's ok if you are just inviting each other's children, but you might encounter more resistance from class mates in school etc. I try to make my 'party bag' something a bit less disposible, so I've given poundland bucket set, with a few sweets, so at least something which can be reused. This year we are doing craft and making some fairy houses, plus they will get homemade fairies and sweetcones with some little 'fairy' sweets - mini smarties, mini marshmallows etc. The fairy houses will probably sit and gather dust(or maybe fairy dust), but they will have hopefully had fun making them.
In terms of presents for birthday child, mine like getting something from a friend, and will often say 'I remember x gave me that for my birthday', however you could operate a present list, or parents could club together to get one bigger present from a group of friends. I don't think my girls would like that, however if that was what you as parents all willingly wanted to do and you each gave a set amount; the children would get used to it, if 10 friends gave a fiver each you would get #50 for something substantial. Again works while you have control over their friends, but might be more tricky when inviting random classmates, as some parents might not appreciate it, you would need to make it optional.
If it is any comfort, the parties get smaller, only having 3 friends for 7th birthday.