If younger people have truly been priced out of the market, then the only solution is to add more housing supply. Clearly you have some aesthetic issue with “half-million dollar McMansions”, but in terms of livable space and maintenance costs they’re much more land-efficient than a 1950’s 2/1 starter home.
Moreover, putting cheap loans in the hands of resident-homesteaders will decrease the tendency for speculators to maximize square footage, and encourage building of the type and size of housing that best meets the needs of residents. Existing “McMansions” would see prices fall as a result of the added capacity.
We can quibble over what type of housing needs to be built, or the best way to incentivize that, but the only real way out of the housing crunch is to build more housing, and the best way to do that is to encourage building over buying.