I've flown on many routes out of LGB (JFK, SEA, PDX) and all are almost always full or close to full. I haven't flown to Vegas or SFO but would if I had to. I think LGB serves a need for more leisure travelers. Lots of Disneyland travelers will fly through LGB versus LAX--I've seen both transcon and West Coast travelers do this. But at the same time I also see a lot of business travelers going through LGB based on the Mosaics boarding flights.
I have flown through BUR too and think it's incomparable to LGB. It's much older and has nowhere near as much waiting area space as LGB does. I also think LGB is more geographically centralized and easy to get to locally. BUR is good for folks in the Hollywood area--I know of several actors who commute on this route from JFK.
My opinion would be for them to keep a presence at both airports as I think they serve different markets. I would also be open to them doing a cost/benefit analysis to the cost of setting up an E190 base at LGB for the underperforming A320 routes.