In the old days, we didn't use multiple Unixes by choice (mostly)
One of the possible reactions to the fading out of multi-architecture Unix environments is to lament the modern hegemony of 64-bit x86 Linux and yearn for the good old days of multiple Unixes and heterogeneous Unix environments. However, my view is that this is false nostalgia. Back in the days, most people did not work on or run multiple Unixes and multiple architectures because they wanted to; they did it because they had to. In fact sensible places usually tried hard to be Unix monocultures (a Sun SPARC monoculture, for example), because that made your life much easier.
The reason that there was a flourishing bunch of Unixes back in the days and people often had so many of them was simple; there was no architecture or hardware standard, so every hardware vendor had their own hardware-specific Unix. If you wanted that vendor's hardware you pretty much had to take their Unix, and if you wanted their Unix you definitely had to take their hardware (much as with Apple today). Unless you and everyone else in your organization could stick to a single Unix and a single sort of hardware, you had to have a multi-Unix environment. Even if you stuck with a single vendor and their Unix, you could still wind up with multiple architectures as the vendor went through an architecture transition. Sometimes the vendor also put you through a Unix transition, for example when DEC changed from Ultrix to OSF/1, or Sun from SunOS to Solaris.
(There could be all sorts of reasons that you 'wanted' a vendor's hardware or Unix, including that they were offering you the best price on Unix servers at the moment or that some software you really needed ran best or only on their Unix or their hardware. And needless to say, different groups within your organization could have different needs, different budgets, or different salespeople and so wind up with different Unixes. Universities were especially prone to this back in the days, and were also prone to keeping old hardware (and its old or different Unix) running for as long as possible.)
Once there was a common hardware standard in the form of x86 PC hardware, the march towards a Unix monoculture on that hardware was probably inevitable. Unixes are just not that different from each other (more or less by design), and there are real benefits to eliminating those remaining differences in your environment by just picking one. For example, you only have to build and have around one set of architecture and ABI dependent files, remember one way of doing things and administering your systems, and so on.