Using Photozone as a reference it looks to me as if the 70-300L cannot quite fully resolve 15 mp on APS-c when wide open at 70 mm, let alone 18, but the OP never said he was using this lens wide open.
Good point, but then again I didn't say so. To repeat myself - and we could probably just ask the op :-p ... he's bound to have used the 70-300L wide open at some
point. And if he'd had found it lacking, he probably wouldn't have used it as his reference what a good lens is to him (see below).
I disagree with your last sentence because the 'very best' in resolution terms doesn't have to be 'the most expensive'. These high mp asp-c cameras need really good lenses, and Canon now provide ones that are up to the job without being expensive; look at the 40/2.8 and the new EF-s 24/2.8.
Come on, this is self-explanatory, isn't it? Of course I was talking zoom against zoom, prime against prime, we all know you can get excellent iq if you use an older manual prime or any newer generation.
In terms of the 70-300L being wow, compared to the 18-135 on my body, it's wow :-) There are certainly times when the image isn't quite as sharp as I would expect but I put that down more to user error and a bit of post processing can usually help. All you are doing by showing comparisons on how sharp it is on FF is making me envious! I usually shoot it at 5.6 so then I can treat it as a constant aperture lens across the zoom range.
This is just what I do with the 70-300L on crop, and it's fine this way and has a very nice bokeh.You really only realize the difference with fine details, apart from the thinner dof on ff.
I mostly shoot horses and focus on their eyes - and here there's a visible difference crop vs ff. And of course to even better lenses like the 100L (which is an example of a lens that is just as good on crop).
Of course, as you wrote, if you downsize and the details happen to respond to the usual sharpening algorithms, no one see a difference. Btw that's why I didn't stretch my budget to get the larger 70-200L.