Many of the bags look like ones that would be under a seat. That's the one I would snag too--laptop and camera.

The majority of handluggage is in overhead compartments, especially larger stuff. It's not a 100% solution but it's cheap and easy to implement.

That would actually waste WAY WAY more time as each and every person will try to open the lock, and fail.

Then some of them will really fight with the lock refusing to leave.

Not if there's a giant LED-lit "LOCKED" next to the handle, which is explained by crew before take-off. Even if they ignored the crew, they'd see the lighted "LOCKED" next to/on the handle.

Mitigating against that exact misunderstanding was part of my suggestion - otherwise you could just lock them with no lights/explanation, but then you'd run into the problem you describe.

That doesn't stop people from panicking (e.g. "My stuff!") and attempting to break the lock. In the NTSB report, people were arguing with the flight crew over whether or not to leave their luggage during evacuation.

Edit: I'll add that I was in a retail store (employee) that was evacuated by police, due to an armed gunman. People were still worried about the office chair or shredder that they wanted to buy. An officer had to basically get up on a counter, and hold up his gun and badge and say, "Everyone out! Now!"

On the way out some guy was trying to ask me if we would be open later because he (apparently) really wanted to get this office chair he was looking at.

And even with a locked message, each person will still try and open it anyway. (You've never disbelieved a sign and tried something for yourself? You've never said "I have to see/try that for myself, even though someone told you what was/did/will happen?)

It's human nature. You have to work with it, not against it.