I think that attitude was fine pre-internet, but these days we are all sharing a network and there's an analogy that can be drawn with public health policies. App stores are one small part of the strategy to help reduce the number of machines that end up as part of a bot net or other similar uses.
Why is that? Why does fear always win? Your argument is based on fear. Why do we favor fear over rationality?
You would choose a dictatorship over a democracy if it meant that you felt safe. This where that line of thinking leads.
You'd rather be safe than free.
Do you look at drunk driving laws as an irrational concession to fear?
The slope isn't nearly as slippery as you seem to think it is.
We got by just fine in the 80s and 90s without App stores. So much innovation came out of that world which established the dominant players who are in control now.
And you don't even get security with App stores. As we already know with all the malware found in Google Play apps.
App stores are simply keeping the big guys big and small guys small with few exceptions here and there.
It's just another walled garden that keeps the little guy out and keeps players like Google and Apple entrenched.