Anyone who disagrees with a leading academic's definition is free to sail off in a gunboat and fight them for the right to call their accent a dialect, or their dialect a language.
Been done before.
All David Crystal was doing was providing a linguistic definition, so that people would be clear what they were talking about (ha!)
I find, on the whole, that this assists rational discussion and prevents participants from rampaging off-topic and down blind alleys.
It wouldn't have mattered if Prof Crystal had called accents 'fish pies' and dialects 'apple charlottes' (or whatever).
Well, it might to him . . . it's his definition.
But it's not my personal fault.
I've said before that I couldn't give a toss whether you describe what's spoken in Little Hicksville-by-the-Haystack an accent or a dialect.
Except that it has already been defined academically which precludes the need agonise over which term to use. Or should.
As I said, sail your gunboat round to Bangor if you want to argue the toss.
You might take in some actual Welsh trad and ditch the MOR shit in the sea.