I’ve got tons of apps in my App Store’s update queue, some that I haven’t updated for a couple months and they still work perfectly. Uber is one that comes to mind. They keep changing the UI while I keep using the same version from ages ago. I don’t bother updating anymore, it’s just noise.
I do update banking and critical apps (say, VPN clients, PDF reader), though.
In the consumer app cases, either I'm hitting somebody's backend with wrong data, which fails and so I update the app, or more frequently, those whiny apps with almost daily updates are just a WebView shell to some website.
Otherwise, what a hacked iOS sandboxed app can do? If there's an exploit to escape the sandbox, like in the WhatsApp case, we have a way larger issue and I wouldn't expect a random hacker to waste such an exploit that could be better targeted at Jeff Bezos or so.
Other exploits are for system apps (Mail, Safari, etc) and are handled by OS updates.