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Keeping the older kernel until you're sure the new one works (which you can't check until you reboot and use the newer kernel) is a good idea. After that, there's no particular reason to keep the old kernel. But there's no particular way to automatically detect whether the new kernel "works"; it could be that it boots, but causes a particular piece of hardware to stop working because of a newly introduced bug, for example. So making the process of removing old kernels manual is unavoidable.
