A problem with using old OmniOS versions: disconnection from the community
One of the less obvious problems with us probably never doing another OmniOS upgrade is that I'm clearly going to become more and more disconnected from the OmniOS community. This is only natural, since most or almost all of the community is using recent versions; as time goes on, those versions and the version we're running are only going to drift more and more apart.
(It's true that OmniOS r151014 is an OmniOS LTS release, supported through early 2018 per here. But in practice I expect that most OmniOS people will be running the one of the more up to date stable releases instead, since they won't have our upgrade concerns.)
Being disconnected from the community makes me sad, because the OmniOS community is one of the great parts about OmniOS. There are several dimensions to this disconnection. First, the more disconnected I am from the community, the less I'll be able to give back to it, the less I can contribute answers or information or whatever. Giving back to the community is something that I would like to do for all sorts of reasons (including that I plain like being able to contribute).
Obviously, the more distant we are from what the community is running the less the community can help us with advice and information and all of that if we run into issues or just have questions about how best to do something or what the community's experiences are. At best they may be able to tell us how things would look or would be done on a newer version of OmniOS. Of course, some things only change slowly, but I suspect that there is only going to be more and more of a gap here over time. I don't want to put too much weight on this; I'm very grateful to the help that the community has given us, but at the same time it's not help that I think we should count on and significantly factor into our plans.
(To put it one way, community help comes from the goodness of its heart and is best considered a pleasant surprise instead of a guarantee or an entitlement. I don't know if all of this makes sense to anyone but me, though.)
Finally, I'll just plain be paying less attention to the community and drifting away it. It's inevitable; more and more, community discussions will be about things that aren't relevant to our version and that I can't contribute to. If people have problems or questions, I'll only have outdated information or more and more uninformed opinions. That's a recipe for disengagement, even from a nice community.
Having written all of this, I think that what I should do is build one experimental OmniOS server to keep up to date. It doesn't have to use our fileserver hardware; for a lot of things, any old server running OmniOS will serve to keep me at least somewhat current. As a bonus it will provide me with a platform to test things on the current OmniOS version (whatever that is at the time).
(We have enough spare SSDs for our current fileservers so that I could take the test fileserver and build a system SSD set for the current OmniOS, just so I have it around. We did this sort of back and forth OmniOS version testing during our transition to r151014, so we actually have a template for it.)