> Any arguments to support this claim? What's the difference then?
The "micro" in "microservice".
Microservices are meant to do one "micro" thing well, whether it's image hosting or credit card transactions or supplying the content of a tweet or whatever.
A monolith does all the things, or most of the things. It's not "micro".
You don't need arguments to support it, these are just the definitions of the terms. It's semantics.
A monolith can't be a single microservice because it's not micro.
In my opinion it is too literal understanding of the concept and there is no universal borderline in size or scope.
It is common in practice to find quite fat microservices because their maintaners decided it doesn't worth to have yet another network communication latency for a specific case.
The main problem here is to be able to effectively debug and maintain several communicating services: to have a distributed tracing facility like Jaeger, centralized logs collection, schema registry and so on.
It may seem compex to set up and maintain at first, but once you have some experience it does not add much toil and cognitive load.
But having the infrastracture in place to effectively work with microservices gives you the freedom of system design choices. It does not force you to make microservices very small.
> there is no universal borderline in size or scope
There doesn't need to be. It's applied in specific contexts where the distinction is clear for that context.
You said "A monolith is just one microservice." But it's not, because they're literally defined as opposites. You might as well say "the color white is just one shade of black." It's not adding anything helpful -- it's quibbling over words.
> It's not adding anything helpful -- it's quibbling over words.
That depends on the context. If I come to the paint store I would be surprised if I'm offered either black OR white paint. As If I can not buy and use both, depending on my scenario. Black and white are generally not the same thing, but they are not mutually exclusive in your practice.
(and by the way comparing sizes to color is a bad analogy. the better one would be comparing a big ship with a set of ships. So I state that I can have a fleet of ships, some of which are also big. And one big ship is also can be considered a temporily outnumbered fleet: if it has means to cooperate I can easily add more smaller ships when needed, instead of builidng yet another deck on the main ship).
My point is: microservices vs monolith is a false dichotomy. You can use both approaches at the same time, since each one of them has its own advantages.
If your infrastructure is ready and you know how to manage microservices, than you can just start building monolith unless you decide that some particular part is better be separated due to some reasons like context isolation, separate deployment, separate scaling etc. It will take you a day for the first time. And an hour for the second time and further.
However if your "infrastructure" is just local files with logrotate on a bare-metal machine, if you don't know how to reuse the common code base for different deployment units or if you do not know how to do backward compatible protocol schema evolution, then bad luck -- you just do not have a choice when it comes to designing a new functionality. You'll have to add the new feature to your monolith despite all the benefits you could reap by separating it.
Microservice-ready infrastructure is all about having the capability to separate parts of logic easily when you want to. It neither dictates you to break your monolith right away nor to build only 30 lines of code services from now on
The "micro" in "microservice".
Microservices are meant to do one "micro" thing well, whether it's image hosting or credit card transactions or supplying the content of a tweet or whatever.
A monolith does all the things, or most of the things. It's not "micro".
You don't need arguments to support it, these are just the definitions of the terms. It's semantics.
A monolith can't be a single microservice because it's not micro.