Personally, I own most of Omni's software, but I won't be upgrading to new versions when they come out.
OmniFocus was once the cornerstone of my productivity. It was expensive and I didn't use the scripting aspects of it, but it provided value. The Apple ecosystem didn't bother me too much, but it was a pain, and I was doing more and more work on my Windows machine and going to my phone to add/update tasks was creating friction.
They came out with OmniFocus web, which would allow me to solve the issue... but it's too damn expensive. I've already spent ~$175 for licenses to OmniFocus for Mac and iOS, now you're asking for $50/year on top of that.
The competitors are hungry in this space, and Todoist offered more for less. I switched to their premium plan, which is cheaper than OmniFocus web itself, let alone the full subscription and includes clients for everything, plus a web-only client as a last resort.
The same pattern holds true for the rest of their software lineup. I own most of it except OmniPlan.
TL;DR. Omni isn't providing value compared to its (paid) competitors, to say nothing of FOSS.