They ought to be at least somewhat concerned that they have less brain for their own walking around the house.

Ah, right. I suppose that would depend on the exact mechanisms, involved, yeah.

Are children who have imaginary friends found to have subnormal cognitive development?

You don't know? It's loss in "utility". When you have an unknown item which, out of the items that you know of, most closely resembles a mushroom consumption of which had very huge negative utility, the expected utility of consuming the unknown toxic mushroom like item is also negative (unless totally starving and there's literally nothing else one could seek for nourishment).

So please provide evidence that this feature is shared by the thing under discussion, yeah?

The cases I've heard which do not look like people attention seeking online, are associated with severe mental illness.

Source? This doesn't match my experiences, unless you draw an extremely wide definition of "attention-seeking online" (I assume you meant to imply people who were probably making it up?)