I just don't get this. First, you're not forced to upgrade your server to Yosemite. If it's working fine as is, you can leave it alone, at least for a while. Clearly, there comes a point where you have no practical choice - new hardware not supported, need support for new features or whatever - and then you're in for $20.
But come on: a very very few years ago, you'd have been talking $150 for the base OS, *plus* several hundred $$$ for the server software. No free upgrades then - maybe a big discount for upgrade vs new. At that time, if you'd suggested you could have an entire server software infrastructure, supported by Apple (or anyone else for that matter) for $20, you'd have been laughed at. Even the open source, free to install options available (red Hat and the like) charged more than $20 if you wanted any kind of formal support.
Where Apple lets us and itself down is that it's just not clear to people that OS version and Server app version go together. This, and many other discussions, started when someone found that out *after* the 10.10 upgrade had identified the issue. *That's* a thing to be annoyed, frustrated or whatever about.
I'm also struck by our distorted way of valuing our time. Even on a modest salary, $20 presents a very small amount of time - and yet we're all comfortable spending hours or days installing, testing, reviewing logs, diagnosing, posting and responding here etc etc etc.
The fact that, for the most part, you can drop $20, do a backup, double-click and check some settings, with a very good chance that you'll have a working server environment upgraded to a new version is worth *far* more than the cost. In most (any?) other equivalent, we're into weeks of evaluation, testing, configuration, roller etc.
Of course, if we're sensible we're going to be testing, reviewing etc before we double-click anything. And Server isn't the heavyweight, major enterprise solution that some of alternatives are. But compared to the under the hood, spanner and screwdriver effort involved in alternatives, we should be delighted that Apple's taken out so much of the manual labour and *for only $20*.