I think that it is good to view generalization as a tool. Like everything in software engineering, it is not an exact since and is influenced by many factors etc. But for me a guiding rule is: "When it is really clear that something will change in the future, then it makes sense to generalize. It is clear enough when you were willing to bet one week of your salary on that the need for generalization arises within X time." (where X is something less up to a year).
Mainly because this communicates intend to future code readers: "Beware, this thing will likely not be alone soon".
Unused abstraction makes reasoning about the codebase harder.
Meanwhile, if you can figure out small reasonable interfaces with potentially multiple implementers, then all the sudden things get a lot easier to deal with.
1. Engineering the generalized solution, then
2. Carrying the cognitive burden of this new indirection layer, and finally
3. Ripping out this premature -- and so likely wrong -- abstraction
Premature abstractions are like alcohol: fun borrowed from the future, and paid back later with interest. If unavoidable, choose the abstraction most narrowly scoped to your actual code base, not a future one. Humans suck at prediction. Ask me how I know.
As a push back ask them to raise an issue to request the interface. In the meantime fix the immediate problem only.
The primary function of an interface is of course to swap out implementations. The first and most common use-case is generally for tests. You can do it other ways too, but they are usually more invasive.
That said, it’s easy to add interfaces later (which is one reason it’s a good abstraction). I would still defer it until needed.
I don’t think your opionion is an outlier among people who have experience in maintaining software. But a lot of teaching make it seem as if abstraction is a goal in itself.
A driver for unnecessary abstraction is the idea that “it will be more difficult to abstract later” not realizing over-designing is a cause of this problem in the first place.