I do not understand: why did you stop being pro-urbanist? How does urbanism stop the middle class from accumulating wealth?
I am not sure that this phenomenon is unique to cities, in fact, but rather an inevitable endgame to the idea of owning land in perpetuity. It creates a permanent class divide.
Of course that opens the question: why should real estate be a continually appreciating asset? There are other countries where it isn't.
Over time, real estate just is inflation - if it's more than inflation it eventually ends up infinitely expensive and unaffordable, if it's less than inflation it leads to cheap-as-free.
Arguably the second is more desirable from a human standpoint - but the first is where financialization leads us.
How does urbanism lock people out of owning real estate?
This can be overridden but it's hard. Most cities with good economies have unaffordable real estate.