Re: This is all too often the refrain from non-Windows users.
"It isn't cheaper when you factor in training, migrations from current systems and support + support contracts (with the latter often being a significant cost even if the only perceivable benefit is having someone to escalate to/shout at)."
That's what Windows addicts are saying, but in the real world employees have no problem working with Macs or even Linux boxes. We have a number of clients that moved away from Windows, and guess what, support costs have dropped notably. Which even IBM found out after they went for Macs instead of Windows boxes years ago. Or the French Police and MoD (which use Linux).
Lots of places have moved away from MS. Just because you didn't look doesn't mean they don't exist.
Just looking at the regular borkage Microsoft puts towards its users - including highly silly stuff like repeated cases of printing not working or links to all applications getting deleted, these problems don't exist on other platforms. It's all this shit that only MS shops have to deal with.
"Then there is the oft-overlooked aspect that many many businesses run on Excel. They have third party plugins from very large vendors (SAP, being one example) that they need to do day-to-day business. And there are simply no alternatives to these in the OSS market."
I understand, but if your business is so highly dependent on a crutch from a single vendor (SAP) that binds it to another, constantly changing tool from another vendor (MS) who has a very poor track record when it comes to software quality then you really should have a very hard look at the dangers for your business that come from this dependency.
What happens when MS makes another change that breaks compatibility with that plugin? Can your business still do the work?
"Like it or not, the corporate world almost entirely relies on the Microsoft stack. "
True, and that's one of the reason we see so many hacked and ransomware'd businesses again and again. Using the same, lacklustre software stack that's build with toilet roll cores and glue makes for a great target for attackers.
And because, especially in larger businesses, getting hacked or losing customer data rarely has any direct consequences for the responsible leadership, not much does change.
"Show me an open source equivalent of Exchange - not the email component but things like calendars etc, that can be synched/shared across devices etc. (Not saying they aren't out there - asking the question)."
Ever heard of Google Workspace? Zoho? NextCloud?
Heck, even iCloud can do that.
Exchange on premises is dead, it has already been killed off by MS, so with MS you're already looking at MS365. For which there are alternatives.
"The simple fact is that if it really were cheaper and quick-and-easy to do, an awful lot of places would've done it."
The simple fact is that many places actually have done it, and aren't looking back. Actually, the only place which did go back to MS was Munich, and that's because because of some severe mistakes made during their Linux migration, in addition to political meddling.
But yes, making the change means someone with balls need to have a look at all the critical software dependencies that have developed over the years, and develop a proper transition strategy which makes sure all aspects of the business are covered. With risk-averse IT leadership that often only knows Microsoft, that's admittedly a hard sell. Peter principle and all that.