Totally agree... the release of the new Mac Pro did not in any way lessen the usefulness nor the value I get from my “obsolete” Mac Pro 5.1!
Regarding probability of failure, yes it could happen any day just like any other product, however, my recent MBPs had way higher failure rates than this old Mac Pro!
Plus, whenever it happens we can still find spare parts, and listen to this, we (users) can easily replace those parts, to get it up and running in no time (I cant say the same for my 2017 MBP which, after several failures, Apple replaced it with a 2018 model - after 5 weeks of downtime!!!)
As for Catalina, yes it’s cool and all, but if the apps I’m using don’t require it (unlikely for another few years), I’m perfectly happy with Mojave.
I don’t see the rush to dump the old Macs and upgrade unless you need to have the newest tech (for whatever reason) and can afford it...