It's more of a guess than a conclusion. Because the tunnel is linear, the failure of a single skate affects the entire line, so running, say, 100 skates concurrently would make overall failure 100 times more likely.

One option is to make the skates incredibly fault-tolerant, but that seems to go against the simple / cheap ethos that this system seems to be going for.

Just to make this more concrete, if your skate platform breaks down just once every 10 years, then banging 100 skates in the tunnel will see this breaking down at about once a month or so.

If you have a flat failure curve, sure. If you have the more traditional bathtub curve, after the initial problems, failure rates should be much lower.