They do, but like all forms of cannibalism, it is unsustainable as a sole, long term source.Spiders eat their young.
They seem to be able to produce enough little spiders to feed the big spiders until the biggies get too big to breathe, and the little spiders eat the big corpses.
That's an ecosystem.
The spider-race could thrive in that panic room for millions of years.
And generally, it's the young that eat their siblings until only the strongest of a clutch survive. I'm sure that things change in desperate times, but feeding and mating still require energy and there will be diminishing returns with. Otherwise, it's basically the same concept as perpetual motion.