Microsoft isn't giving up on assistants (as per the article), its just going down a different route, more AI focused. This is sensible because Cortana is a flop, nobody uses it, there is no need to leave it running.
I'm certain Apple has the same plans around AI, however people still use Siri all the time. It's not a flop, so it may as well stay running as more and more AI works its way over to it or its replacement.
It's really weird to me that anyone would want it shut down when it's so easy to just not use it.