What you're seeing is a huge washing up of tuna crabs, Pleuroncodes planipes, which has created "red beach" all up and down the California shoreline, as well as other states. It's not an uncommon occurrence farther South, and is quite frequent in Baja and along the Mexican coast. There are some interesting scientific theories as to the cause, the changing weather and tide patterns caused by El Nino being one of them.
Although the most recent large scale event in your area was the first since 1983, you'd be interested to know that the species itself was first discovered right there in Monterey Bay!
From the Sanctuary Integrated Monitoring Network:
Pleuroncodes stranding events are rare occurrences in Monterey Bay and usually coincide with ENSO events. The species was first discovered in 1859 during a mass-stranding event in Monterey. Further stranding events in Monterey occurred later in 1959 and again in 1969. Dr. Steve Webster, senior marine biologist at the Monterey Bay Aquarium, noted that when present, seagulls were feeding on so many crabs that they could not get airborne. Pleuroncodes are also a source of food for fishes, rays, and pinnipeds (seals and sea lions).
Most of the crabs are dead, and those that are alive will likely die naturally, get eaten by birds, or be stomped on by people. Apparently when dead they cause a terrible smell, and some people come through with "raking equipment" gathering up all of the crabs, both dead and alive. Where you live, though, that's illegal, as there's a delicately balanced eco-system, and all organisms are protected, so you may see more live ones than people in other areas.
Also watch for sightings of blue whales. They feed on the tuna crabs, so when these things happen, they may come closer to the shore than usual. Scientists at the Monterey Bay Aquarium reported sightings of 20 Blue whales within a week of the arrival of these crabs on May 26, 2016, more than the usual number. These are being closely monitored, and steps are being taken to protect them before they become stranded on the beaches.
As for your efforts, I don't think they were futile. Leaving them alone is a certain death knell, and some sources say that they're doomed either way, but I'd be tempted to turn a few over and at least give them a fighting chance. I did read that these crabs are ill equipped to function on the sand, and don't really know how to find the ocean. That may have been why they seemed confused, or perhaps it was the shock of what they'd been through! Some people are gently moving them back into the ocean. If you had time, in addition to turning some over, you might want to do that. They have a tendency to walk backward, so you may want to face them towards the sand rather than the water, though I don't know how important that really is.
Obviously, you can't save all of the living ones you find, so that's up to you. If you choose to flip or move them, know that they will pinch a bit, but don't bite or excrete any venom, so it's a safe, if time-consuming, task. People should never eat tuna crabs, whether living or dead, as they're generally full of toxins from what's in their diet.
My feeling is that any effort you make is worth it. I have a personal bias in this area though, so you need to take that with a large grain of salt (very bad pun intended). My husband and I don't ever let anything die if we think it can be saved. When we come across flies, bees, beetles and other creatures lying upside down and flailing about, we always turn them right side up, which at least gives them a fighting chance. Many times we're pleasantly surprised to watch them take off. We also save baby birds that have fallen into the swimming pool, but that's a different topic!
You shouldn't feel bad, though, if you decide to just let them be. I couldn't find evidence that they suffer more acutely on their back or their stomach, so do what seems best for you, and be gentle on yourself if your tender heart hurts for these beautiful little creatures. There are reasons for these things which we humans can't understand. For instance, this could have happened to feed some birds that are in the area at this particular time.
My best answer to your question is that while it's a stretch to think many can be saved, it's not a foregone conclusion.
Even though you didn't specifically ask for more information, this question intrigued me so much that I did a lot of research and found an abundance of resources about both the crabs, and these incidents. In case you're interested, here are a few:
Mother Nature Network, an article from May 26, 2016, reporting the incident on Del Monte Beach. This one has video of the invasion, including some dying crabs, but also includes a pretty video of the crabs alive and swimming at the Monterey Bay Aquarium.
Eco-Watch, an informative article from May 20, 2016 with pictures and videos.
Scripps Oceanography Institute in San Diego, good article from a similar incident in San Diego in 2015, with links to more information.
Oceanlight, pictures from the National History Photography blog. Most of these are on beaches in San Diego and Ocean City, some from Baja.