I get this feeling that Arc is the language that is just going on by momentum.
Further, I don't think Arc, PG version, is as well-designed as you think.
For example: "code is spec". Apparently the code itself is to be the specification for the language. However, this brings up some questions: for example, does the fact that (ssplit '(1 2 3) 0) return ((1) (2 3)) part of the specification, or is that a bug? http://www.arclanguage.org/item?id=8450 .
Or how about the cute little fact that the PG 'map function can be safely recontinued when you capture the continuation in the mapping function if the given structure is a list, but is not safely recontinuable if the given structure is a string? Is this a deliberate design decision, an oversight, or something you don't specify and so any other implementation can do what they want?
And while tagging allows the programmer to arbitrarily create new types, those new types don't get along well with the builtin functions. I can't build a vector type in PG Arc that will work with the builtin functions. I can't build a quaternion type which I can multiply with a scalar number using '* .
These are design points which have been belabored for years; Brooks in 1975 attacked using "implementation as spec", pointing out that it limits future implementations by forcing them to always use the old implementation, for example.
If arc is continuing by momentum, it's a very different kind of momentum than the momentum languages like Java, Python and Perl survive on. They have momentum in that they have many large and complex libraries and programs written in them which are being actively used to the point where rewriting said software in another language is no small task; and hence most people are interested, not in making a new and better language, but in making the old language better.
Arc has none of this. The reason why it continues is because the people using it are enamored of it for its own sake, not because they depend on it. This is both a blessing and a curse. A blessing, because it means Arc is free to change and improve; a curse, because it means Arc is incomplete and a moving target.
This is one reason why it is important for you to be here: some simple things like the problem with 'splice are clearly bugs, but other things are not. For example, the fact that '(1 . 2 . 3) reads as '(2 1 3) is just something drained accidentaly from mzscheme's reader or is part of Arc? Since you've written Arc, you're the only one to really know this.
We can only suppose.