Personally I don’t think there’s business in commenting system precisely because there are many options already.
This discussion here is framed as “competing with Disqus” but you would also competing with JetPack, Lifefyre, Intense Debate, Facebook and many, many others.
Even if this was 2008 and those options didn’t exist, economics don’t support $20/month comment hosting business because acquiring users is not free.
You can’t just sit and hope users will come to you.
There’s only so much traffic you can get by promoting in forums like this one.
Lacking some genius (and free) marketing strategy, you won’t get enough users to support yourself.
Disqus (and everyone else who matters in this space) went with free product with ads because of the economics.
The free product was a marketing strategy and initially it was very costly, which is why they had to raise $10 million in total (https://www.crunchbase.com/organization/disqus#/entity) and go big or go home.
They were lucky and got big but that’s not a route you can replicate.
Philosophically speaking, if you plan to do something different from everyone else (i.e. do a paid-only business vs. generous free tier with ads or upsell to paid tier as everyone else who’s successful) then either you understand something they don’t about this space or you don’t understand something they do.
In this particular case I think they understand the economics of this business and you don’t.