I'm sure there are plenty of people who don't watch NCIS
and are very interested in Agents of SHIELD
. I'm one of them.
My understanding is that the whole reason they scheduled AoS opposite NCIS was because they saw an opportunity to draw in a demographic that NCIS wasn't dominating, and thus become competitive with CBS in that time slot. And really, why wouldn't it draw a big audience? Not only is it guaranteed to draw in the fiercely loyal Whedon fanbase, but it's a TV series that's canonical within the universe of one of the biggest ongoing film franchises of our time. So it's got two
built-in audiences (although there's already a fair amount of overlap between them).