This statement is false. The availability of third party appstores does not in any way affect the security of your parents devices. Just instruct them not to install other appstores.
And yes, time and again, fans of handcuffs are proving that they are willing to trade other people's freedoms for their own convenience.
Think of it as a church. You being a zealous Jehovah's witness shouldnt stop others from praying in other churches, not does it break your relationship with your JW cell.
One plus phone was a mess because the product is mess. It wouldn't get any better if users weren't allowed using any other apps. For example, my wife's parents use Ubuntu desktop for many years, with next to zero maintenance. Ubuntu does have an ability to install other apps but through its software repository, but it is made in such a way that an unskilled user just can't break the system. A simple switch buried in the settings enabling third party repositories would be enough.
And yes, time and again, fans of handcuffs are proving that they are willing to trade other people's freedoms for their own convenience.