No one is forcing them into an ecosystem.
Reality number 1. They are the most secure phones on the market. We don't need to rehash the malware debate here, but Android suffer pretty badly.
And they have clearly addressed your concerns about app security here:
"..risks from installing software from unknown developers that are not subject to the Apple Developer Programme requirements, installing software that compromises system integrity with malware or other malicious code, the distribution of pirated software, exposure to illicit, objectionable and harmful content due to lower content and moderation standards, and increased risks of scams, fraud and abuse."
They also lose their rights on open Internet when they go to certain countries. Women also lose rights when they go to certain countries. But let's worry about downloading the latest porn from your favourite side loading app store shall we?
Perspective people, it's downloading an app when you have been out of the country for 2+ months!