Well yes, perhaps the expectation of a young couple being able to live in a detached house with garden esp in a city is somewhat unrealistic, actually, the 'want' part is surely not so much a new and unreasonable idea, I imagine having a big house and land has always been aspirational throughout history and having a garden/land particularly if you can use to grow your own food was probably even more desirable pre-war than it is today. It's definitely not something only spoilt millenials want!
But the concept of a property ladder, starting with a small flat then trading upwards through renovation and capital growth, that is really quite new actually as an idea, it probably started with my boomer/post war generation who on the whole have done very well from the housing boom, possibly at the expense of today's younger people. It was very different in recent history, e.g. My lower-middle class war generation parents bought a terraced house with (small) garden as their first home post-marriage in their 20s for some absurdly tiny sum, 5 shillings and sixpence or something, and that wasn't unusual for the time. It wasn't a huge house or anything but they didn't need to 'trade up' at all, they stayed happily there for 50 years+. My early 20th century working class maternal grandparents rented rather than bought but again their first (very modest!) home was one they stayed in pretty much for life, my paternal grandparents were a little richer but again didn't do a lot of moving around, I think they bought an initial house when they were first married and then maybe did move to somewhere bigger a few years later when their income increased due to grandfather's business dong well but definitely not multiple moves over the course of a lifetime. Going back even further to Edwardian/Late Victorian great grandparents they were a mixture of in service/living in tied accommodation, working class or blue-collar workers who rented and wouldn't ever have owned property at all and farmers/agricultural workers who lived rurally so very different housing situations to what's normal today and the idea of any of them climbing a property ladder is absurd.
I'm not saying that today's generation aren't at all unrealistic or unreasonable, sometimes they can be and in many ways they have it much better than their ancestors did, no-one today would expect to live in the squalor of my grandparents' tenement flat where they brought up 4 kids with no hot water, no electricity etc, I just think it's also a bit silly to criticize millenials for not doing what 'our' generation did as though it's a self-evident historic truth that that's what sensible people have always done to end up living in lovely comfortable big houses by their middle age and make loads of dosh to boot. When in fact our generation is the one that's the blip in terms of having been able to do that relatively easily...