Disclosure up-front: I've been in the payments domain for 15+ years 10 of which have been in Indian payments ecosystem.
From a user's stand point this is a fair criticism. However, if it helps to mitigate your bad user experience, it helps to know the larger context in this this exist.
The government of India's first, and mostly sole, priority is to build a payment network and get most of its citizens adopt it. This is an unprecedented challenge at multiple levels which very few know, let alone appreciate.
Back when UPI was being built, the digital infrastructure was shitty, smart phone penetration was not much to speak of, banks were unwilling to support this, merchants didn't care, most of the citizens didn't even have a digital/online identity let alone bank account -- just to name a few challenges. Overlaid on that is the incredible diversity, scale (1300+ millions), political diversity and so on. Multiple and parallel mega-initiatives had to be carried out (Adhaar for identity, Jandhan for bank accounts, NPCI to pull together all the banks, overhauling subsidies system) besides indirect push through demonetisation.
For the Indian government, payment infrastructure was a means to several ends such as equitable distribution resources, plugging the subsidy leakage through corruption.
Now in all this international users' use case is so low down the priority that no one would even bring it up, let alone acknowledge it. Even if we go by just numbers, international transactions are so low in comparison that it's less than round off error.
With that, let me try to address your frustration points because not all of them are unique or specific to India.
> Indian POSes expect pins and most international credit cards do not have one so they get auto-decline
As others have pointed out this is mostly due to US cards being behind the curve in adopting better security standards.
This is a feature not a bug and neither is it specific to India. Between acquirer, issuer, network and regulator any one could demand the POS to enforce a PIN failing which merchant is expected to be liable for fraudulent transactions.
> Some slightly smarter POSes will try to do things like Verified by Visa and usually there are so many bugs in implementation that things never gets through.
Verified by visa (VBV) is one specific implementation of 3DS (3-domain-secure) for online payments. You seem to be confused between POS payments and mobile/web payments. But this is a valid observation/concern. There way too many systems involved in the transaction chain and coupled with India's not so reliable internet infrastructure it's a recipe for shitty experience. Though it's improving fast.
> One of the challenge I give to non-Indian folks is buy Internet access on international airports in India. It is impossible unlike rest of the world
The best way to circumvent is to buy airport lounge access. Almost all the lounges accept international cards so you should be fine. It's bit expensive but not so much in $ terms and the lounges are in fact quite nice with free food/drinks nice seatings etc., :-)
> The worst thing is that to even get in the Indian payment system you need govt issued citizenship documents and wait for approvals.
This, unfortunately, is the by-product of Indian government prioritising Indian citizens as I explained above.
> Indian websites accepting online payments are usually extremely poorly designed
This however is fast improving, especially if you use native mobile apps. But even then you do have to contend with 2FA with is downright horrible on mobile device but there are some auto-otp-read features that reduce the pain.
There's been a talk of dropping 2FA requirement for low value transactions but I think it's still to be done.
> can't handle International credit cards at all
This, again, is a feature. The fraud rate is so high on international card that it's just not worth it to enable them. Note that the issue here is on the issuing side i.e., stolen US cards are dime a dozen and shockingly they just work out of the box thanks to next to no fraud control on them. There's an option to go through Stripe/Paypal etc., but then their rates are very high (again, due to high fraud rate of US cards) that it doesn't make business sense.
> So imagine you come to airport, have working International plan but you can't use it for payments or anything because the entire system assumes you are an Indian citizen with documents, all government approvals done and have a mobile phone number in India.
This too is a by-product of what I explained above. That said, within International Airports your card should just work fine on POSes. Because those POSes are configured to accept them as, well, they in fact deal with more foreign issued cards than Indian ones. So I'm surprised to hear that that's not the case. Something doesn't add up here.