The dependency on logging in with the profile you use to purchase it a severe problem for me. I have to switch profiles a lot, between school, work, and my personal one (yes, all 3 use Google). So even though it's just me sitting on one single computer, I can only use the paid upgrade 1/3 of the time. 

As an alternative business model, you could consider doing Patreon like the extension Sprucemarks and its creator does:

https://chrome.google.com/webstore/detail/sprucemarks/fakeocdnmmmnokabaiflppclocckihoj

https://www.patreon.com/nightmode/