Giving Apple the benefit of the doubt here (which is a significant caveat), I'd like to think that they modelled out various scenarios and looked at growth rate

Giving Apple the benefit of the doubt here (which is a significant caveat), I'd like to think that they modelled out various scenarios and looked at growth rates to know that the year 3 scenario you envisage rarely occurs. Or, they could've just picked $1 million because it's a nice round number and looks good in a press release.