Yes, that's been my experience for 'dbus' and other changes. It seems to only be a kernel update which pulls the rug from under its own feet for some use cases, requires a reboot to resolve those issues.
Yes, that's why I suggested a 3-tiered classification scheme above.
1) Kernel upgrade - reboot ASAP. Basically, when the kernel has been upgraded, you are now in the land of UB (Undefined behavior) - where anything can (and usually will) happen.
2) Some other integral (by "integral", I mean not really user-visible - not a front-line application) part of the system that could start to fail, but if it does, the effect is likely localized (unlike with the kernel) I think this "dbus" thing falls into that category.
3) Some piece of software that is "front line" - and which won't be working completely right (or at all) until you reboot. This area you obviously can do whenever you like, because you know what is going on.
The general principle here is that the more "under the covers" something is, the more it behooves you to do the reboot sooner than later, since you (as an end-user) will be less likely to accurately diagnose whether or not things are failing.
If a kernel update didn't pull the rug from under its own feet I would accept not needing a reboot notification.
Right, although it would still be nice. At some point, one or the other of us ought to "apt install" that "needreboot" thing mentioned by jojopi and see if that works well or not.
I also like the idea suggested by another poster that the old modules be kept around (and used) until the next reboot. That would solve the whole problem.