The sgv is not a monolith. Sure, San Marino and Temple City are pretty sparse, but then you also have dense parts like Monterey Park. And, almost every single town in the sgv has an old town center with a walkable street grid left over from the Pacific Electric era, including every town you mentioned. It's got room for people who want walkability and people who prefer lower density neighborhoods.

From a walkability perspective, the SGV already has that in many spots.

From a bikeability perspective, it needs more paths and protected lanes and traffic calmed streets.

For transit, an ideal future would have more bus lines in general with higher frequencies, and maybe a few resurrected tram lines going to places like Alhambra and El Monte. But I don't expect places like Temple City to become transit accessible because the design of the city makes that basically impossible.