"iOS market share has declined tremendously since 2010, from approximately 80% to approximately 20%."
Not so. IDC data: iPhones shipped in 2010: 47.45m. Total smartphones shipped worldwide in 2010: 303.9m. iPhone share: 15.6%.
iPhones shipped in 2014: 192.6m. Total smartphones: 1.295bn. iPhone share: 14.8%.
As a percentage of all mobile phones (including featurephones), iPhone shipments have risen steadily from about 3.3% (average across 2010) to 11.8% (average first two quarters of 2015).
I don't know where you got your numbers from, but they don't match any I recognise. (Equally, if you're using "market share" to mean "install base" - don't. They're not congruent.)
> iOS market share has declined tremendously since 2010.
Has it? It looks to me like most measures have current iOS smartphone marketshare at or above 2010, but down from a peak in 2011-2012. Android is a lot higher than in 2010, having eaten most of everything that isn't iOS
iOS market share has declined tremendously since 2010, from approximately 80% to approximately 20%.