DSSSL is not Scheme, it's a domain-specific language, which solves nearly all of your objections that aren't conjecture. Domain-specific languages are obviously much more amenable to optimization. Such a stylesheet language can be trivially cache JIT compiled stylesheets, and because they're more expressive, you'd see more reuse across pages, and possibly even across sites. The bandwidth savings are non-linear.
No. I don't believe this is true. With CSS as a declarative language, we can do global optimizations that are much harder to do than with a general programming language (especially one that's as hostile to static analysis as Scheme!)
> Do you agree or disagree that such a layout language could supplant some of the uses of JavaScript over the years? This too seems to be an undeniable yes.
Sure, but that's not worth slowing down so many Web sites for.
> And you can claim gzip is good enough to eliminate any space savings a more expressive language would yield, but the fact is people still minify their JS and CSS for significant savings, which means even small differences matter
Sure, but it's not worth trading off the rendering performance.
> a precompiler could perform advanced common subexpression elimination passes
Not with Scheme, it sure can't! You can do those dynamically, but not statically.