I am sure one more number could squeeze in there.
Unfortunately for archviz or any work with large scenes, the Outliner really doesn’t help with it’s +99 (also shouldn’t it be 99+ with plus at the end?) and it just looks like short-sighted design aimed at working with small scenes with only a couple of objects.
I agree that having three digit counter there could be very helpful.
Now the + thing, I see it this way:
+99 -> More than 99
99+ -> 99 more
Now, I don’t thing it’s " short-sighted", TBH inside a collection you have multiple collections, and sometimes we could have more than 100 objects in one collection of course, but generally big scenes are very partitioned in several collections, and once you have all the trees of the same species in one collection, it does not matter too much if you have 99 trees or 9999 trees of that species, so it all depends on how do you manage the scene.
With that said, let me agree with you once again, that having one more digit couldbe useful in other situations, even being able to configure the amount of digits you want in that counter, from settings.