Walkability, density, and population are all distinct features of an urban setting. You could have a highly zoned, non-walkable city that is very dense. You could have a lightly populated city that is not walkable because of sprawl (see: all the countless cities defined by "stroads").

Sprawl occurs as a result of tax policy, transportation infrastructure, zoning, and other factors.

One could envision a version of LA with less vacant lots, less surface-level or curbside parking, less single-story fast food franchises, etc. One could envision an LA with an efficient transportation like Hong Kong.

It is completely possible to achieve livability with a high population and high density. Indeed, that generally will correlate with more cool stuff to do.

Reducing population would by no means necessarily lead to walkability or livability.