Having been through this experience about 3 or 4 times in the past 4 decades, I will say "Yes", it is indeed a failure, because as I did (and probably TweetBot did), I built a whole business around the assumption that creating an 'add on' app was a solid, viable, long term business.

When those other vendors decided to either (a) close down or (b) stop sharing APIs and critical integration details or (c) change their architecture considerably at short notice or (d) build exactly what you have developed internally themselves, then you will be stuck with pretty much nothing. (Yep, I have pretty much had all of the above happen to me).

The taps will be turned off with little notice. You will have to go and tell your staff that they are out of a job and don't need to come in Monday or ever again because you haven't got the cashflow to pay them any more. You will have to go and try and break the lease on your office space with your landlord. You will have to tell the bank that you are suddenly unable to service any business lines of credit or your cards. That is NOT my definition of success.

I would far rather plod along with an app that I have built in relative obscurity, knowing that I can put strategies in place to deal with the regular ups and down of the market. Writing a remora app just means that there IS a definite finite shelf life for your product, only you won't ever know when that is.