I had never heard of scratch before but I find this line of thought generally backwards. Maybe that's what carmack is getting at but it seems to me that early "exposure" doesn't really matter as much as people want it to.

There is so much software all around us these days, any curious kid who's interest is piqued by it will find a way in to the hobby and maybe to a career.

There was a period where having a computer in a classroom was a big deal because it might be a kids only exposure to one but that's not the case today (not everywhere, I know). Kids pursue their interests if they can and software is very easily pursuable these days.