This is the elephant in the room that most comments on this page miss. Office may be hard to replace, Teams maybe even harder, but the real pain comes when you touch identity and access management. The usual initial optimism that "yeah but [insert solution name here] does this, problem solved" dissolves very fast as you start going through the inventory of requirements for managing users, devices, authentication, etc.
It's not just the technical hurdle which maybe you'll whip your admins into finding workarounds (-keep praying that your admins don't leave because it will be painful to find replacements who understand and can maintain the spaghetti pasta monster your infra ended up being-). In overall non-technical organizations the user experience always ends up hobbled even just by asking people to keep track of multiple identities.
MS is still entrenched because they give a turnkey solution with Eeeeeverything™ and your CTO doesn't need to struggle with any uncertainty. SaaS made it so easy to just "outsource" everything to MS, they'll be responsible and accountable for operations, infra, security, processes, etc. Even less headache for your C-level people. See no evil, hear no evil, you pay MS to take the shit and your job is safe. If you throw a stone out the window you'll hit someone with general "MS administration" skills. And users are usually familiar with MS tools, Windows, Office, so they aren't bothered (you hear a lot of complaints about Teams on HN but not so much from normal users). So this covers the tech, the skills, and the UX.