Mobile platforms are far more secure than desktop computing software. I'd rather do internet banking on my phone than on my computer. You should too.
We can make operating systems where the islands can interact. Its just needs to be opt in instead of opt out. A bad Notepad++ update shouldn't be able to invisibly read all of thunderbird's stored emails, or add backdoors to projects I'm working on or cryptolocker my documents. At least not without my say so.
I get that permission prompts are annoying. There are some ways to do the UI aspect in a better way - like have the open file dialogue box automatically pass along permissions to the opened file. But these are the minority of cases. Most programs only need to access to their own stuff. Having an OS confirmation for the few applications that need to escape their island would be a much better default. Still allow all the software we use today, but block a great many of these attacks.
Both are true, and both should be allowed to exist as they serve different purposes.
Sound engineers don't use lossy formats such as MP3 when making edits in preproduction work, as its intended for end users and would degrade quality cumulatively. In the same way someone working on software shouldn't be required to use an end-user consumption system when they are at work.
It would be unfortunate to see the nuance missed just because a system isn't 'new', it doesn't mean the system needs to be scrapped.