Apple - AppleCare engineer says Macs need weekly reboots b/c "kernel expires". What's that mean?

Documentation on this does not exist.

Having worked as a Certified Apple Engineer (CAE) I can tell you that something has most definitely been lost in translation (for want of a better word) from the original source (i.e. the engineering team) to the AppleCare adviser to you.

While it is arguable (opinions vary greatly) that regularly rebooting a computer (i.e. any computer) is beneficial, there is certainly no kernel expiry going on. And, as others have already stated, rebooting a Mac is only required for a limited number of updates/upgrades (e.g. firmware updates, OS level updates, hardware upgrades, etc).

I regularly have various Macs (including MBPs) running 24/7 for weeks/months at a time without any issues. So, being told that you don't reboot your computer often enough, which is necessary because the kernel expires, is nothing more than a figment of someone's imagination. Imagine if all those Macs being used as servers needed a weekly reboot - Apple would be the laughing stock of the IT industry!

My recommendation would be to research (and maybe post a separate question about) the issues you're having with Safari's CPU usage.

As others have said, this doesn't sound at all like a reasonable response from Apple, and likely something is lost in translation. That said, monthly (or nearly so) kernel panics is definitely something "out of the ordinary". The fact that this has persisted across 3 different machines indicates either you have the worst luck with buying flaky hardware, or something common has been transferred from machine to machine.

I'm assuming you haven't been transplanting RAM chips or HDDs from laptop to laptop, so that leaves either software or some external device. If every new computer you've used TimeMachine or Apple's built in transfer software to migrate your environment, it's possible that some software issue is being propagated each time you upgrade machines. Unfortunately, the only "quick" way to validate this is to boot from a clean installation and use that until you're confident the problem no longer persists. If you have an external drive you can use, this is a possibility for you, but otherwise, you would need to back everything up and completely reinstall without restoring the backup, which may be more effort than you want to take. It's possible it's hidden in user preferences in which case creating a new user account and using that might help, but in my experience it's about 50/50 whether it's a user level issue or a system level issue.

Assuming you don't want to go through those steps, I would try to determine what each kernel panic has in common. If they all seem to happen and coincide with your safari issues, then that narrows it down to a likely issue in safari. You can either try using a different browser for a few months and see if the issue disappears, or try eliminating any plugins/third party addons that you have installed in safari. If the issue persists, runaway CPU usage strikes me as runaway process likely due to some software bug interacting with some ad or other tracking/javascript that is on the sites you're using. Once you've eliminated all of the various plugins as the cause of the issue, you could try installing an ad blocking plugin, preferably one that can block javascript too and crank its restrictiveness to max. This may make some of your browsing less enjoyable (and may even break some sites you use) but these sorts of plugins also allow you to turn off the blocking on a site by site basis, so over time you can relax the restrictions until you find the site that's causing the issues. From there you will have a better starting position for determining exactly what's giving you trouble.

If the kernel panics occur even outside of safari usage, you may be looking at your safari problems being caused by whatever is causing the kernel panics, in which case you want to tackle that before you try to figure out safari.

External devices can also cause kernel panics if they're not behaving correctly. So if you use an external HDD or something else commonly plugged into your USB or other expansion ports, try seeing if not using that device clears things up. Even something like a USB thumb drive can cause issues. It's not a common thing, but then again, neither is monthly kernel panics. Alternatively, if you always use the same port for the device, you could try a different port to see if it's an internal hardware issue, but this is less likely given that the problem has persisted across multiple machines.

I'm not sure who you talked to at Apple, but Macs, like all Unix-based devices, do not require routine reboots, except for certain updates.

As for the kernel, it is the heart of the operating system. There can be only one™. It should have nothing to do with your problem here. If Safari has runaway CPU usage, I'd try deleting its caches first. While in Safari, click on the Safari menu, select Preferences, select the Privacy tab, select "Manage Website Data...", then finally select "Remove All". Note that you'll have to log in again to some websites, but you won't lose anything crucial.