My understanding is that the problem isn't Office itself, but the cloud and online collaboration features. So saying "just use Word95 or Open Office" is in no way a solution.
Domain controlling is perhaps something not easily replaced, but the rest is just fluff in my opinion. Teams might be the most useful app of all the bunch.
Seeing other cursors in a Word document was a fun at first, now it only induces endless rage and while neat, it certainly isn't a requirement.
There is a reason why Azure and AWS based cloud systems are pretty much a standard. There isn't a solid alt.
O365 Exchange keep you free from worrying about keeping your server up to date. O365 Teams allows for not only communication but collaboration. Teams integrates with SharePoint allowing for project sites. O365 Sharepoint & Onedrive allow for extremely flexible data storage and data shares. With all the permission options you'd ever need. You can move your AD Login's user folder to use your personal Onedrive account, a highly effective tool.
There just isn't good options to replace O365. Google's GSuite is a crappy alt for Admins that couldn't figure out 365's complex admin consoles.