I was replying to Razib on this comment & thought that instead I would turn it into a post. I would also like to caveat my thoughts:
I have a habit of generalizing since I am now more used to social media posts than blogging. As always I’m very happy to be wrong and these are simply my thoughts and observations.
The fact that my wife is a Sindhi from Chennai (among other things) gives me an additional window into “other” parts of South Asia long inaccessible to the average Paki. As an aside my late eponymous paternal grandfather was actually Kakazai but my grandmother was an Urdu-speaker from Amroha so that culture was instead transmitted to the next generation. So I speak both as an insider and an outsider to this Hindustani cultural complex that I’m commenting on. Not fully in but neither out & just enough on the margins to make it interesting!
How do Desis from any part of the Subcontinent connect? Let’s say as a thought experiment we take the extremes of the South Asian desi region; a Tamil, Pashtun, Nepali and Bengali walk into a bar. Now none them may know Hindustani or they may speak it with a very heavy accent but that is their common link and bond, which would even inflect the English that they speak to one another (English usage of course depending on their socio-economic strata). If they didn’t want to watch a Hollywood film they could conceivably agree on a Bollywood film. UP, in that way, is the beating heart of all South Asia.
All of these cultures are radically different from “Hindustani” culture (for want of a better term) but there’s enough familiarity with it, which makes it a civilizational links of sorts (or a cultural lingua franca that underpins Desidom).
A culture that hasn’t been touched by Hindustani culture or is far removed from it (both Afghanistan and SE Asia were at times part of the Indic cultural sphere but it’s hard-pressed to consider Burmese, Dari-speakers or even the Hindu Balinese as Desis) doesn’t get absorbed into desiness. Whereas Nepal has sufficiently desi touches even though its people do look very different (most of the Nepalis I’ve met in my limited experience do seem more East than South Asian).
As a final point to a very great extent Hindustani culture has been deeply influenced by the colonial project as part of the divide & rule strategy (I don’t want to go into the Hindi-Urdu controversy hence why I’ve used the neutral term Hindustani). There is a reason why that, despite their very different geographies, both the successor states to the British Raj (India & Pakistan) depended on this culture as a nation-building project for their diametrical ideologies.
There were of course severe limits to the Hindustani language project with riots in South India in mid 60’s and the independence of Bangladesh in 1971 but even so the march of Hindustani as a core component of Desiness remains unabated especially with the rise & rise of Modern India & Hinglish.