Yes, I know, but Urdu written in devanagari isn't called "Urdu", it's called "Hindi". The only thing reifying "Urdu" at all is the choice to make a cultural affiliation with the Arab world.
Especially around Lucknow, there are certainly vocabulary differences, and some very minor grammatical ones (matching numbers in verbal phrases, some nouns changing gender, but overall yes, I see your point). I would be interested in your opinion of what Kamleshwar's novel Kitne Pakistan was written in or Munshi Premchand's early work published in Devanagari?
> I would be interested in your opinion of what Kamleshwar's novel Kitne Pakistan was written in or Munshi Premchand's early work published in Devanagari?
You've exceeded the bounds of my knowledge, but I can provide some generalities. Writing systems can have no effect on the, um, "identity" of a language, since it's quite possible for people to be illiterate. So to me, saying something is written in Urdu, or saying something is written in Hindi, are exactly the same claim about the language involved. I have been taught that Urdu is written in Arabic letters and Hindi is written in Devanagari, and since that is the only difference between the two, iconic cultural works of Pakistan, if written in Devanagari, would fairly be described as being written in Hindi -- they have all of the defining characteristic[] of Hindi and none of the defining characteristic[] of Urdu.
The existence of geographically-based variation in vocabulary and grammar, especially minor variation, doesn't make for a claim that the languages are different. In Georgia (and, so I'm led to believe, other parts of the American South, but I can speak to Georgia from personal experience) it's possible to combine modal verbs, which is grossly ungrammatical in standard American english. Using just examples from country music, it's also possible in southern dialects to use verbs that don't exist in SAE ("I ain't [...]") or to conjugate verbs in impossible ways ("He don't [...]"; "We was [...]"). There is no serious case to be made that they're not speaking english in the South, though (Jamaica would be a more interesting case).
The only other place I'm familiar with where the alphabet (term used loosely here) is held to, under its own power, define the language, is China. I actually had a dispute some weeks ago on HN with someone advancing the argument that the "Chinese language" is the writing system, and any spoken language is a dialect of "Chinese". Despite its total incoherence, this is essentially the "party line" in China, and a very common view among people educated in the Chinese school system. So here are some examples I used there:
1. If the english sentence "How old are you?" is written as 多老是你？ (pronounced "how old are you?"), english has not become a dialect of Chinese.
2. If the mandarin sentence 你多大？ is written "Ni duo da?", mandarin has not ceased to be a chinese language, nor has it become a dialect of english (or turkish, or any other language written with roman characters).
3. Considering two illiterate Chinese people, one of whom speaks mandarin and the other hakka, is it correct to say, since they're both illiterate, that neither knows any language at all?
It should be easy to see that choice of writing system is orthogonal to what the language is that's being written (or, in example 3, not written). Chinese politics dictate that different languages with 0% mutual intelligibility be made to appear "the same". What we see in Pakistan is the opposite impulse, that the same language be made to appear different for political purposes. But this has no basis in reality; Urdu and Hindi, with their 100% (!) mutual intelligibility, are the same thing. Arabic letters don't make Urdu different from Hindi any more than they make it similar to Arabic.
Ok then, why do I regularly encounter Hindi words like "saphalta" and "viruddh" that I have never encountered written or spoken anywhere in the Pakistani mediasphere and have to resort to a dictionary for (when there are perfectly good 100% mutually intelligible equivalents "kamyabi" and "ke khilaf/mukhalif hona/rokna")
You know, for my whole life people have been complaining to me that I use words they don't understand. Words like "quixotic", "propriety", and "fee schedule" (those aren't invented examples; they all happened recently enough for me to remember). They've never suggested that my vocabulary actually means I speak a different language, though.