I'm afraid the memory management scheme might cause the Nimrod ecosystem to end up with too many libraries that require GC, and since the building blocks are already using it, why not use it yourself?

And what would be the problem with that? The number of application domains where even soft real-time GC is inappropriate is very, very small. And designing freedom from garbage collection into a language (other than the simple option of allowing manual allocation/deallocation) is not cost-free, either, and can weigh down the design, too. You can optimize your language design for having garbage collection or for not having garbage collection, but if you try to do both at once, a language design that specifically favors one or the other will likely beat out the compromise solution for the case it's optimized for.

And yes, that means that Nimrod (or any other language) won't be perfect for everything under the sun. Which is fine: there's nothing wrong with having multiple programming languages, each of which is better suited for a different task.