These "monopoly" accusations are pure nonsense.
Apple doesn't have a monopoly in the smartphone market, they don't even have a majority market share (less than 30% globally, in fact).
And Apple certainly doesn't have a monopoly in the (secure/rich/whatever) messaging market. In most countries, WhatsApp is vastly more popular than iMessage, even on iPhones.
Saying that Apple's behavior here has anything to do with a monopoly is like claiming WhatsApp has a "monopoly" on sending WhatsApp messages, or Facebook has a monopoly on publishing Facebook posts.