My records show that I paid $4.99 for "Insync Pro" in April 2013, and that you have supported this software license and replied to my tech support emails promptly ever since. For the period April 2013-December 2016, this is equivalent to a $0.11/month subscription, clearly not a sustainable business model for you!
I understand why you need to add on new paying customers to be able to be somewhat profitable!!!
Google Drive is a service that Google does not develop to its full potential (at least, not the client side), and it is appreciable that you are filling that niche.
My only complaint is that I have encountered several times a standard "race condition" synchronizing problem—in which the app changes the timestamp of a file that is currently being edited, which prompts the app doing the editing to say "A more recent version is available on disk, would you like to read it." This is a standard problem with all synchronizing mechanisms. I have emailed several times about it, with no avail.
What I want to highlight is I understand how a company that has not figured out its business model can be taxed to reply to service requests like that.
I am happy to pay an upgrade license. But as somebody who works in software, I don't know if the "one time payment" is a good business model. As people have mentioned on this page, it is normal for a user to say "Well no new features have been added, why should I pay?" when they do not realize Google's API changes so often that just keeping up is a full time job!
At any rate, please clarify your new pricing policy and I will be happy to support you. Thanks for creating software I have been able to forget exists for the past three years, as it has almost never given me trouble.