Assuming your 5-year support scenario is accurate (and it's as decent a guess as any can make here), five years of productivity for an initial cost of $1600 - $1800 makes reasonable business sense to me. I've usually worked my Mac Pros for 4-6 years before replacement, and usually at an initial cost of $3000 or so.
The type of work I do hasn't changed all that much, so the need for the newest/fastest model simply isn't there. My primary need is for hardware that can run Mojave (at least), and perhaps Catalina and the next one or two updates so that I can stay up to date with my software (Adobe Creative Cloud and a few others). It's cool if I have to ride an OS for a couple years in order to keep from buying new hardware. I've done it before (I'm still on Sierra at the moment, and I rode Snow Leopard for about 5 years prior to that).
Yes, I know the 5,1 can run Mojave and Catalina, but the way I've got my system set up, upgrading would be a nightmare. I've looked into it, and concluded I simply don't have the time (or inclination) to waste on it.
So it's not more speed I'm after. That said, the 2013 trashcan in any configuration is still faster than my 5,1 (going by Geekbench scores which are arguably not real-world tests, but are better than nothing).
I like the idea of avoiding the Apple Tax by buying used. I'll be honest, six years ago these computers had a lot of power but were overpriced for my needs at $4000+. But now, that amount of power, while not impressive by today's standards, is enough for me and is priced very reasonably at $1800 or so.
