They could save a good chunk of the 69 USD they spend on Digital Ocean servers.
edit : To clarify, What I meant was that to serve 55k users, perhaps using more efficient languages like C# or Java, would allow them to run on lesser spec machines. And as those users scale up, to say a million users a month, perhaps savings would also increase, if using more performant languages.
But scaling it to a million users, say. A python app would consume way more resources than an app in C#
Realistically, the author could probably serve 10x-100x more users (your million users) and still stay on the cheapest droplet with python. As you can't go smaller than the smallest, changing language can't be justified on a cost basis.
Finally, the performance differences between a well written python app and a poorly written c# app are probably not that large. The developer probably started with python, because that's what they know, and wouldn't be able to match the code quality in c# without significant effort.