Hmmmmmmm hypothetical huh???

Well yes and no. I know our situation is different from your "hypothetical one" in that my older boys are teenagers so very much in the "parents suck, my friends are everything and I want to be as far away and spend as little time as possible with my family" stage.
DH and I joke whenever we go somewhere or do something that it will just be "our weekend family"

We have a 4 bedroom house and nobody shares (well except me and DH) We have forfeited the garage and turned it into two bedrooms and a rumpus room kinda thing, so everyone has their own space and they also have somewhere to go with their friends to hang out. This has been done with blinds and bookshelves etc as makeshift walls nothing permanent.

Holidays are crazy expensive as "families" are apparently two adults and two kids so to be honest an extra child hasnt affected that bit.
I dont really know how much more expensive going from 4 to 5 would be normally because our number 5 was Lady, so that changed the rules and made her one very expensive little girl That was very much my doing and had she been a little boy it would have just been another spud in the pot as we had everything from S already.

We had a 7 seater car anyway so that wasnt an issue.

I love it, sure I would like more room, more money, more sanity, more time but as Guy Sebastian says "If you only think of things that you havent got, you could have it all and still never have enough"

Oh and I promise they will eventually sleep