I focused on that somewhat in this thread because that was what you initially asked about, but note that only my first response to Swift's creator's justifications even mentions that. I do think servers moving from GC to RC would not be very advantageous, but not for that reason (or at least, like you said that reason only applies to a minority).
The biggest reason is that while some server side applications are very latency sensitive, many (most?) are not. For most websites, significant (i.e. collections of older generations) GC pauses are going to be relatively infrequent and having a few requests slow down just a bit for less than a second is not a huge problem. In these cases (which if they aren't the majority, they're still close) throughput is more important that latency, which is where a GC shines. And ease of programming/maintenance is likely to be more important than both.
As far as "helping you reason about your object graph", I don't see the argument about how forcing you to RC helps you do that at all. All of the extra reasoning is centered around something that would be otherwise irrelevant. Even in cases where you do something that is not amenable to GC collection (e.g. events/observers that keep objects alive which you didn't intend to), it seems of little benefit to have to constantly think about all of your other references at the same time.
> GC pauses are going to be relatively infrequent and having a few requests slow down just a bit for less than a second is not a huge problem
I respectfully and strongly disagree. Outlier performance issues are a huge problem when scaling businesses.
Additionally "ease of programming/maintenance" is exactly an area where, overall, Swift may have some major advantages over Java. Without even further debating the cost-vs-benefit of declaring dense graph relationships, I believe you've conceded that this a small part of the overall picture.
> I respectfully and strongly disagree. Outlier performance issues are a huge problem when scaling businesses.
For server-side GCs, a major collection is well within the bounds of incidental network latency, and happens far less frequently. Outlier performance is a huge problem, but from what I can tell outlier performance specifically due to GC pauses is an issue in far fewer cases (one that comes to mind is servers for certain financial services).
> Additionally "ease of programming/maintenance" is exactly an area where, overall, Swift may have some major advantages over Java.
That's true, but these advantages are uncorrelated to reference counting and they diminish if you choose to compare with a language that has aged better (C# comes to mind). If you're really concerned about high-performance, low-latency server programming and you are willing to switch to a less common language you'll be much better served by something like Erlang, which is specifically designed for that.
Erlang's memory model is fairly similar to what I have described here, but still runs into some issues that I feel could be solved by giving more control over where exactly things are allocated. It also resists any kind of mutable state; I agree that mutable state can complicate concurrency heavily, but having an option to use it sparingly would help (process dictionaries are too limited).
My point is basically that just like GCs aren't the solution to every problem, ARC isn't either. I think a language that allows for both will beat both Java and Swift handily when it comes to making both easy things easy and hard things possible.
The biggest reason is that while some server side applications are very latency sensitive, many (most?) are not. For most websites, significant (i.e. collections of older generations) GC pauses are going to be relatively infrequent and having a few requests slow down just a bit for less than a second is not a huge problem. In these cases (which if they aren't the majority, they're still close) throughput is more important that latency, which is where a GC shines. And ease of programming/maintenance is likely to be more important than both.
As far as "helping you reason about your object graph", I don't see the argument about how forcing you to RC helps you do that at all. All of the extra reasoning is centered around something that would be otherwise irrelevant. Even in cases where you do something that is not amenable to GC collection (e.g. events/observers that keep objects alive which you didn't intend to), it seems of little benefit to have to constantly think about all of your other references at the same time.