Is Iron Man costing them customers? Probably not. But it is costing them customer diversity, which is what you need to expand your overall customer base and continue to grow as a company.
Right now they are stagnant. And they are effectively putting most of their eggs in one basket, so if something unexpected happens on that side of things they would take a huge hit. Particularly as they have been alienating a lot of past customers with a combination of either ignoring former key licenses, or announcing products that then never seem to see the light of day, all while yet more Iron Man variants, Marvel movie characters, and Johnny Depp characters take first, second, and third priority over everything else.
They have Batman and Star Wars, two of the biggest licenses in the world, and they are barely using them (especially the latter). That just seems like madness to me.