Do these theories spur gentrification, eliminating affordability?
Indeed. Cities that have become walkable struggle with gentrification. But in most cities that still need to become walkable, in fact, the housing in the heart of downtown is typically subsidized housing. The sort of housing that needs to be encouraged downtown is market-rate housing, and city governments need to take an active role to introduce additional, less affordable housing downtown just to get the right balance. Eventually, if they are successful, gentrification will become an issue and then there are tools available to protect against it.
What can the general public do?
It's what the public is doing, in many cities in America. It's to clamor for bike lanes, slower-speed streets, and for streets that serve all their users, not just motor vehicles. We all know how to be active within our communities. We can be active around this issue.