A common zoning change here is based on street frontage for semi-detached homes - the new ones are still 3-4BR, just attached at the garage and with smaller yards. If development required 15m frontage, but then that changed to 12-13m, that would mobilise a lot of owners to take advantage, though obviously others can just stay as is if they prefer.
It usually happens that an $800k lot value becomes $1m, regardless of the state of the house. The owner can then demolish a decades old house, build two places for $600k, sell one as a new home for $800k-1m to finance the build (and costs of moving out during that phase), and end up in a new house themselves. Often they've sacrificed yard that they found annoying to maintain anyway.
The above can be adjusted where it's possible to build 3-4 on a block, or a larger development of apartments.
Zone changes typically allow change, not force it, surely? An owner can just keep their SFH and large yard if they prefer. What they can't always control and often vote against is the composition of their neighbourhood.
What? There is a structural deficit problem. The ship is sinking. Complaining about how "we shouldn't have to change anything about the ship" isn't really a reasonable argument. We live on this ship... we have every incentive to make sure it stays above water.
You might think there is option 4--municipal bankruptcy--but that is just option 2 and 3 combined.
Building buildings somewhere else will not fix your neighborhood.
I don't really think those are the only three choices, though. The government can fail and be replaced with a new one that will shape things up. Then it'll be replaced by another that thinks it's too big and well off to fail, squander it, and fail. That's the typical cycle.