DocHub will be around for a really long time. We have vey long term plans and goals, but for legality of an electronic signature in DocHub, the binding agreement will always be valid regardless of whether DocHub is still an active service or not according to the ESIGN act and several other international electronic signature laws. I would definitely recommend that you save a copy of your audit trail for additional legal backing for any of your sign requests which you can find out more about at this article:
How do I view an enhanced audit trail for Sign Requests & Form Fills?
Even without the Audit Trail (if you are using a free account), your signed documents are definitely legally binding, but the Audit Trail and Audit Mode will just provide even more legal proof and details about the signing of your documents than a free account and will help save you time in court if the other party tries to contest the legally binding document.