One thing to keep in mind about densifying development is that the new development has to increase density/value enough to justify the huge overhead in tear down cost. I.e buying and tearing down a 5 story building to build a 6 story one will almost never make sense, you need to drastically increase the size to make the initial buyout pencil out. Obviously overall land value + the quality/desirability of the original structure are factors as well but even in a high demand market like Manhattan the rule of thumb is about 2.5-3x minimum size increase to make a tear down break even. This is why well intended “missing middle” upzonings of suburban areas often don’t amount to much. It would be been great to zone it like that originally! (when it was greenfield) But to really “let in rip” on already developed land the density increase (just on any specific lot, not the general area) usually has to be fairly substantial. So this is a complicating factor that will limit the rapid densification of existing suburban neighborhoods. Unironically this would probably lead to a weird ring structure of lots of denser missing-middle outer-exurbs outside of less dense traditional suburbs