The Medfly, like the original Oxnard commuter (with SP gallery car - often singular), were political one-shots with no money and no policy behind them. Of course they failed. The Rail Division at the time had other things on its mind, alternating between simple survival and trying to start up a San Joaquin and unload Caltrain on the JPA. Prop 116 hadn't happened yet, iirc.

The Coast route is longer than going down the Valley, especially if somebody listens to The Mouse and resurrects the hole in the hill route. Even going over Tehachapi and thru Palmdale, it's faster than the Coast. SJV route has several larger towns (Fresno & Bakersfield are bigger than anything between San Jose & LA on the coast) for intermediate traffic. Several parts of the Coast are iffy from a long-term standpoint, especially with sea level rising (which is happening, regardless of whether or not you blame humans for it), and would need to be relocated at major cost to make s higher-speed (say, 90-110 mph) operation work. Realistically, considering costs and nimbys, the Coast might see some more improvements (though nothing major) from LA to Santa Barbara, and from Salinas to SJ, but not elsewhere. It's not a LOSSAN - nowhere near the traffic potential, couple hundred miles longer, and lots more costly problems to work through. LOSSAN can cost-effectively be patched up to function at a 100-125 mph level, and to avoid any serious sea level problems for at least 50 if not 100 years. Not so the Coast. And yes, for major transportation arteries (standard engineering practice - 20 year design life - notwithstanding), you do have to plan on 100-year timescales.