I really agree with both sides on this one. Yes, everything will change, and so it should - you will have loads more clutter and mess, you will spend your time differently, and your whole life will be transformed, as it should.
BUT this "fitting in" thing - I actualy do think the child should do some fitting in with you and your agenda as well. It's a balance. If your child doesn't have to fit in with your plans and priorities quite a lot of the time as well, you'll be a slave to their needs, lose sight of yourself and run the risk of raising a person who really thinks they are the centre of everything and the universe revolves around them.
I would never have expected life to carry on the same as before when I had DS - but equally, I'm not going to stop going to parties and gigs, having nice objects and furbiture, having a christmas tree etc. He can and does learn what things he mustn't touch - we all know we have to teach our kids not to touch electric sockets and hot ovens, so we can also teach them respect for our property (obviously excepting some SN situations). He comes supermarket shoping with me and I teach him to help, he comes to a gallery or a gig with us if that's what we're doing, and learns to behave through doing that.
Of course he's a 2yo boy and it's not always a success, things sometimes get trashed, there are sometimes tantrums which I know is to be expected. But I do think it's sad when people have kids and then only ever do things that are designed for kids and won't let them learn about the normal adult world and how to behave in it. I have a friend like that - her DD never goes to the supermarket, on errands with her or to any grown-up occasion. All they do is go to toddler classes, playgrounds and soft play centres and my friend is knackered from squeezing in the shopping and everything else after bedtime. So how will that child learn to behave in a grown-up way when necessary?
I also think it's a shame to come down on a new-mum-to-be with all this cynical eye-rolling and "oh, you'll find out, ha ha ha" (obviously I'm paraphrasing!) just because she'd like to teach her kids to be a bit better-behaved than the horrors she recently had trashing her own house!