We have upgraded to every release of Vmware Workstation since 8.
Usually, there is support for new O/S releases, new features and performance enhancements. We suspect Vmware also adds support in their drivers for new CPU instructions Intel adds from time-to-time that better support a virtual environment.
Having said that, we don't use every update of a new version release. Version 14 is a good example. If you look at this thread here, we ran into a problem with the mouse jumping around after the first update to 14 and none of the other updates seemed to address the issue. So we stayed on the initial 14.0 release. We just started playing around with 15 but waited until 15.0.2 came out to let the initial bugs get addressed. So far, we have not seen the mouse jump around but we just purchased the upgrade today (hence I saw your post when checking for anybody raising RED flags about 15).
As a Windows person, I really prefer the Workstation under Windows approach that lets us have a Window host and then various Windows guests where we do our real work. So, by purchasing the upgrade every year I hope that Vmware will continue the Workstation product and we figure our 2 cents must help their bottom line a little bit 
How often do you upgrade Workstation?
For me this has significantly changed over the years.
Deciding question is the expectations versus risk assessment.
Version 2 beta upto Version 8:
I expected that every new version would be better than the previous one so I installed every new version on day 1.
I also could handle all installer issues myself - in those days I could install a new version even if I had no installer at all. So I never regarded a new version as a risk.
In the days of WS 6.5 I recommended Workstation for small scale productive use
Version 8 upto version 10.0
I had to adjust my expectations.
Since 8.0 I no longer expect a new version to be better than the last one. I rather expect to see a tolerable drop in performance and consider it almost a success if all functions of previous version still work as expected.
Version 10.0
Since that version I learned to be careful. It was a shock to run into a unresolvable issue with the installer with the final result that I had to reinstall my Windows host.
Versions after 12.01
- thats the version I use to call "the last known good one"
Since then I rather hope that the core functions hopefully still work in next version and that the performance still is tolerable.
Since then I tell users that I regard using a VM with monolithicSparse VMDKs as an unacceptable risk if the data is valueable.
Same applies to all encrypted VMs - I only recommend to use them when the content is discardable and not important.
Versions 14 and later
So VMware forces me to buy new hardware ???
And they completely failed with communicating this to their users ??
That change would have looked acceptable if version 12 had already checked CPUs and had send an early message like:
This host runs CPU XY - Workstation may drop support for this CPU in next version.
Even a discussion in this forum some months in advance would have been nice ....
Michael forgive me for being honest - you asked ...
If you ask this question because you want to know if you can sell a new version in 2019 this is my answer:
Personally I would not buy a new version #16 in 2019 unless a future nested ESXi 7 VM fails in WS #14 or #15
I have very low expectations for the next version in case it comes in 2019. Actually I rather expect that it will suck ....
As mentioned before I am afraid that Workstation will go down the drain unless it adjusts the direction very soon.
I probably was one of the best Workstation evangelists that a company could wish for. - but
today I would tell everybody that asks about an update to stick to his existing working setup as long as possible.
I recommend to schedule a full day for an update as it may be necessary to revert it.
At the moment I recommend to use ESXi free version or VirtualBox if someone asks for a recommendation.
I would be glad to see a new version in 2020 though - but only if you use that extra year for a global bugfix overwork.
So many good features that we got used to, silently disappeared during last versions - that trend needs to stop.
Some more free advice for the productmanager coming from a long term loyal user - so I hope you dont mind.
One of the major advantages of WS in the desktop market is the fact that WS comes from the same company that also sells ESXi.
You do not use that advantage at all - if I were in your shoes I would want to establish WS as a valuable ingredient of the vSphere-soup.
And by the way ... I need Workstation for my VMFS recovery work so please make sure that WS will survive at least until I can retire 
... that should do for now - hope it helps
Ulli