I've got also an LG with WebOs, 55UJ634V. I've seen around 3-4 major updates do the WebOs, and the apps I use, Netflix, Amazon Prime, and NowTv are constantly updated time by time. And they work very well so far, for almost two years. The only complaint I have is the Wi-Fi signal reception, is not that great.
I also thought in selling the mibox, but what I'd get? 30 quid? So I decided to turn it into a snes emulator. A pair of snes wireless control from aliexpress, a usb hub, and it's all set.
App availability for WebOS (or frankly, anything that isn't Apple, Fire, Android, or Roku) is fucking terrible. Even if it meets your needs today, you will likely want to use some new service in the future and not be able to because of WebOS.
You bought a tv from LG with webos, thats reason enough to keep it
I recently bought an LG 65SM9000PUA and have been going back and forth between using my Shield TV (I know it's not mibox, but still feel it be comparable) and LG webos. To be honest with you, I find the webos to be /currently/ fine for my needs albeit a bit messy to navigate. I pop on my shield to use GeForce Now and some other apps from time to time.
I'd hold onto your mibox just in case if your model tv stops receiving frequent updates or if an app eventually breaks and isn't fixed.
Android TV integrated is the superior solution
Kodi with a personal collection of movies and tv shows
I've had an LG TV with WebOS for a few years now. The software has been updated more often than my MiBox and has all of the apps I need. The apps are updated pretty regularly too. I've been very happy with WebOS.
Apps and updates. I find that TV makers abandon their software pretty soon after they launch new hardware.