Edit: I suppose in all except the free self hosted one, OpenSign would be the trusted third party, which I guess is more plausible. Unless the paid customers are given something close to root to administrate them. Still, a trusted third party is generally based on recognition. Even if I really dislike a company I eventually acknowledge they're trusted if it lasts long enough, like with ID.me. I didn't use ID.me until it was required for logging into the IRS and now I grudgingly admit that I think it's an extra security check on logging in. So until you're big like DocuSign I wouldn't view you in quite the same way as a trusted third party.
That does bring a question, are your paid customers prevented from going under the hood in such a way that they would also have to be trusted at such a level along with OpenSign?
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This to say I'm open to using OpenSign, because there are plenty of uses where I would be open to using something that doesn't have this "trusted third party at the level of DocuSign" feature. The "digital notary public" analogy is apt. I sometimes sign documents with a notary, and other times without.