1) 5x is a lot, in a deflationary environment. You could assume more, but then you'd need fiat inflation, which might cause the house price to also rise.

2) Anything over 10 years is complete guessing.

3) Houses will continue to be scarce until the house-owning population declines (i.e. Boomers) and frees up real estate.

4) After hyperbitcoinization, most mortgages don't really make sense. At most, maybe rent-to-own plans.