There is that group. Which if you think about it, is a group reluctant to spend.
Not necessarily. When you've worked with these kinds of things long enough, you realize that most businesses are willing to spend money on *something*, and the willingness to spend is often connected to how and where it fits in their budgets. You have businesses much more willing to spend $10/month every month, reliably, then to spend $120 per year all at once. You have some that will spend money on renting things, but not buying things, or vice versa. Don't underestimate the importance of being able to push an expense past the paper-pushers.
Along with everything else, sometimes it's just an irrational thing. I can get a client to agree to spend $10/month every month for the next 5 years, and explain, "this is just what you're going to spend." It works itself into their budgets and they just take it for granted. If I charge them $600 every 5 years instead, even though it's not that much money and it amounts to the same amount, it'll be an argument every 5 years. "Do we really need to spend the $600 this year? Can't we put it off? Can't we just not spend that money?"
Of course businesses don't want to spend money that they don't need to, but I don't think it comes down to "not wanting to spend money" as much as people who want their expenses to be justified, which is pretty much everyone.
Why? Because the spending would be mandatory?
Not exactly. Because it's mandatory in a way that doesn't make sense. It might make sense in cases where people lease a computer. Let's say I lease a Dell laptop for 4 years, and included in that cost were all Microsoft software updates for those 4 years. That makes some sense, because then I'm essentially renting the hardware and the software it requires for 4 years, after which, I'm done with it. However, if I buy a computer, I've bought it, and I expect it to be able to run its basic functions indefinitely, until it breaks.
And that's why I think a mandatory OS subscription is a really awful idea. If you're very knowledgeable about computers, you might not think this way, because you think about how you can install a new operating system, and therefore think of it as separable from the computer itself. However, for most people (and most businesses), it's more of a single unit. You buy a machine, and the operating system is just part of it required to make it work. Without an operating system, the thing is useless. So it's not like Adobe CS or Microsoft Office, which people see as an add-on piece of software, but it's something directly involved with making the hardware that they bought functional at all.
So it's not like employees or heating, which are services that you pay for. It makes sense that you would continually have to pay for it. It would be better to compare it to buying a chair where the seat is designed to self-destruct every month like clockwork, rendering the entire thing useless until you buy a new seat from a single particular manufacturer. It might make sense if you rented the chair to say that you have to pay every month, but if you've purchased the chair, why would you put yourself in the position of needing to pay monthly, for no reason except to satisfy the money-grab of the seat manufacturer?
That being said, there are a ton of vertical applications for Windows that just don't exist for Mac.
My experiences is that you might find applications for either that don't exist for the other. For enterprise, managing a large fleet of workstations is a bit easier with Windows workstations running on a Windows domain, but there are actually management tools for Macs. Most Windows IT people just don't know about them. If I had an objection to Macs for the enterprise market, it'd sooner be that Microsoft Office for Mac sucks, and Apple seems intent on screwing up their file sharing with crappy implementations of SMB.
But yes, I would take your point that in some situations, the TCO for Mac would probably end up being higher. But then, that's what I said-- that the TCO may be higher or lower, depending on "things like user training, what kinds of systems your IT department is familiar with, and what kinds of functionality you need from your computer."