Read your own words "immutability allows for concurrent access" Most GCs mutate (mark bits, semi-space copy, etc.) The immutability of Haskell data structures is an illusion, and from the standpoint of real concurrent access, an expensive one.
The most efficient scheme probably is a "stop the world" system. The regular program might mutate concurrently and the GC could indeed be concurrent, but making them run at the same time is a real challenge.
Of course, people don't like it when their app pauses.
I think it's noteworthy that there never was a big office suite or even a web browser written primarily in a GC system, despite many years of promises.
As for web browsers... every web browser has a GC...
Yes, but Java and GC proponents talked up the possibility at the time. There were various projects to do pure-java office suites and web browsers. The time or two I tried them they were memory pigs and slow.
Lot's of apps bigger and more complex than an office suite have been written in Java.
Many that take take heavy user interactivity? Eclipse perhaps, but IBM had to come up with a custom native-code UI toolkit to implement it. I still considered it too slow to use until the last year or so.
As for web browsers... every web browser has a GC...
Last I looked (a long time ago) my impression was that DOM and Javascript interpreter objects were usually refcounted internally. Did this changed?
If your problem is with pauses during GC, well guess what computers pause all the times. Whenever you move your mouse or type on the keyboard, they generate interrupts that pause the whole CPU and it has to handle them. Those pauses don't seem to be a problem. If the small pauses in GC are acceptable in general usage, why not use them?
If you want hard guarantee, that's what realtime system is for.