According to Mixpanel, the 5S / 6 / 6 Plus together account for a bit less than 10% of iOS devices they see. So it's not as big of a drop as one might expect.
I must be missing something here. Assuming on a napkin that all users roughly generate the same amount of revenue, losing 10% of them will lose you 10% of your profit, no?
If anything, users of really old iPhones are likely to be less spendy so you're losing less than 10%.
It depends on how you're calculating it: if you have fixed costs, then yes, your profit drops 50%; if your costs scale with number of users then you go down 10%.
Profit is revenue minus expenses. Supporting more versions of iOS essentually means greater expenses. So the decision to drop support of older iOS versions is basically the questions of whether or not the additional expense of supporting it is greater than the additional revenue those users generate. For a company like Facebook, the revenue is much greater than expense, while for a smaller company, it's probably smaller.
https://mixpanel.com/trends/#report/iphone_models