They are not trying to push you to Office 365, they are abandoning (phasing out) MS Publisher. Nothing wrong with sticking with Office 2021, I'm still using Office 2007 on one system but just installed Office 2019 on another. I was using Office 2000 up until 5 years ago and MS was sending me security updates the whole time. I use to use Publisher a long time ago but that was only because we actually printed paper news letters. Haven't used it for 15 years because we quit printing paper newsletters.
Google search can suggest some free and paid alternatives if you feel like trying some out but but you have is all you need.