The separate /boot partition is partially a leftover and partly a method of increasing the likely-hood of the install working on a larger group of hardware. Ol

The separate /boot partition is partially a leftover and partly a method of increasing the likely-hood of the install working on a larger group of hardware. Older machine bios'es often had a hard size limit of where on disk they could read boot data. Having /boot be a small partition at the start of the disk meant a greater likely-hood that the new install would boot properly. Newer Linux boot loaders have largely eliminated the need, but old compatibility items everywhere tend to stay stuck well beyond the point where they ceased actually being necessary anymore (partially due to the "if it is not broke, don't fix it" mindset). But the problem you describe is not a failure of having /boot be a small separate partition. It is a failure of the upgrade scripts not cleaning up old, no longer used, kernel files.