So you decide to run your infrastructure on something else. Good luck trying to find a contractor who does anything but Active Directory, MS Office and Exchange. Or try to find admins who have experience setting up keycloak as an alternative identity provider or using ansible to manage the endpoints. They exist, sure, but are much rarer.
If you are a somewhat larger corporation and decide to buy up a shop, they most likely run AD, O365, etc already and merging the IT landscapes will be much easier if you do too.
If you're a small operation of a few dozen people or so, it is soo much easier and probably cheaper to simply sign up to O365 and get the entire suite of email, office, calendar, IM, file sharing, etc, than finding a contractor that manages your IT using all open source software.
Every time this discussion comes up, most people focus on the office suite and perhaps the OS of the workstations, but the business world is much more entrenched with Microsoft products than that.
For the record, I think it's vital to avoid vendor lock-in, and I hope us Europeans manage to do it sooner rather than later, but I'm also afraid many people severely underestimate how hard this is going to be.