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Agree with all major reasons above. I don't upgrade (major version) unless I have to (due to printer driver or software version compatibility). Upgrading may cost you also. Some apps (like parallel desktop) will charge additional upgrade cost for the new OS.
 
As most others have said, what's on your work Mac can be vastly different than what's on your home/personal Mac. My iMac at work has Sierra, and my IT dept. has no interest in upgrading. My workhorse at home (2017 iMac) has High Sierra, because I use an eGPU with a Nvidia card connected to it and we're all still waiting for a Nvidia driver for Mojave. And lastly, my new Macbook Air is up to date with Mojave. All work fine, so there is presently no reason to upgrade the older OS's.

I have to agree with your boss.
 
"Upgrading" an OS at an enterprise/business level is a serious expensive undertaking.

There are multiple applications, both purchased or developed in house, that MUST be certified to work with the upgrade. There are training implications for the user based that must be addressed. The company internal call center must be prepared with training and scripts to respond to the inevitable calls. The IT department must develop, schedule and staff for the rollout. And so forth.

"Patching" an OS at an enterprise/business level is also serious undertaking. The IT department must evaluate these patches similar to above as well. If it is a "critical" patch, IT "may" roll them out after testing and planning. Upgrades and patches always break something in the enterprise world.

Bear in mind, the above discussion applies to organizations with 100's to 10,000's of usually Windows devices. The largest rollout I managed was over 100,000 PCs in 110 countries.

But you say, I don't give a hoot about big companies issues with upgrades and patches. It's just little old me with a Mac on autopilot updates. True too an extent. You are your own IT department.

Upgrades and patches to Macs can have similar negative issues as with a Windows machine. New features to learn, existing features disappear (i.e. Cover Flow), missing drivers (i.e. Nvidia), etc.

The difference here is you are controlling everything with your Mac. In a company, you are not. Now the OP (original poster) of this thread works in a small company and there is a debate about upgrading all the Macs to Mojave. This issue becomes a risk/cost assessment for them. The boss is taking the least risk approach by standing pat. When the risk becomes to high to NOT upgrade, they can go for it.

Ok, yada, yada, yada. My recommendations for an individual Mac user is to NOT blindly do a major OS upgrade at first offering. Give it a revision or two to work out any initial bugs. When you want to take the plunge, plan for a rollback just in case. That requires a backup of the current state and an image copy via software like Carbon Copy Cloner (CCC). Then if you want/need to downgrade back to the original OS you can.

I just went through this process using CCC to restore High Sierra after upgrading to Mojave. Mojave broke my photography workflow with the dropping of Cover Flow as well as driver issues.

(By the way after working with Windows in the Corporate world for decades, I was overjoyed to return my Windows laptop at retirement. I now have a 27" I7 SSD iMac and what a sweet machine. I wish I still had my 1984 Mac as a souvenir as well.)
 
Some of the above responses have noted that better to stick with what came with, then cite how much better Mojave is than HS. So if I bought a computer that worked only with HS or newer, the argument goes, better to stay with a crummy OS (which HS was for a few dot releases) than move onto a better version.
I have worked in both very large and very small organizations. Certainly in large orgs OS version updates were required because of security updates, and changes in the OS that would accommodate the ecosystem of third-party apps that were used.
 
As for me, I always upgrade to the latest (be it beta) to test new features and bug fixes. I am an app developer (not a pro, but as a hobby), and it's important to be prepared for the next OS versions.
 
As others have stated, security risks are the biggest factor because Apple doesn't always provide all fixes to past OS's.

I personally am a slow adopter myself. I won't touch a new OS until it's on at least the X.3 version.

Maybe you could convince him to move to a new OS once it hits the final most stable dot release (usually .5 or .6) right before a new one comes out? (In this current case High Sierra 10.13.6)?
 
OS 10.15 is due to come out in the Fall w/out support for 32 bit apps.
What that will mean to those of us with 32 bit apps that can't be updated to 64 bit, well that remains to be seen.
The folks who have 'Automatically Keep My Mac Up to Date: Install Mac OS Updates' set as their preference
may be in for quite a surprise.

Whole number updates are the worst. Fractional updates can also be problematic and security updates, albeit less risky, are still a cause for concern. In all cases have a bootable clone handy if it doesn't work out as planned.
 
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