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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: