> still survive for centuries as one country
You have drunk the koolaid, my friend.
India became 'India' in 1947. Before that (and even now) it was exactly like Europe ... many languages and cultures and food very very different from each other (ask a Naga what he has in common with Tamilians or Kashmiris), a large shared Hindu ethos similar to how Christianity was a common factor for Europe) and lots of warring kings with their own territories. There was no "Indian" identity.
About Congress ... no defence about their fuckups. But don't be so dismissive about what they accomplished. I say this as no fan of Indira Gandhi.
Remember that India had become free after 200 years of enslavement and harm to our self-image, from being one of the world's richest areas to the poorest, and millions of lives had been torn asunder under partition.
In Congress's era, esp. Jawaharlal Nehru's time, we got a beautiful constitution, the premier institutions and facilities (TIFR, IISc, IITs, FTII, BARC, AIIMS), developed a massive power infrastructure with hydel and nuclear, became a nuclear strong power, developed an in-house rocket and satellite program etc. It had a strong court system; recall that Indra Gandhi was thrown out by the Allahabad supreme court on electoral fixing.
Manmohan Singh was (and is) a learned economist who righted many of IG's fuckups. He led the 1991 reforms. A large large portion of the projects that Modi has lent his beaming face to were fully funded and started in the Manmohan era (Chandrayaan, Mangalyaan, Atal tunnel, UPI, Aadhar, so many airports and highways). The Modi govt has been successful in rebranding all of them in his image, but don't be fooled.
I can give you a huge laundry list of things that are terrible under Modi. The COVID response, the complete takeover of august institutions by deranged Hindus, the complete submission of the media, the complete takeover of massive projects by a few crony friends (a profitable and well-run Mumbai airport was taken over by Adani, who literally had zero experience in running airports, with no formal publicly transparent process). The "PM Cares" fund, which every single govt employee and every corporate was FORCED to hand over money too, apparently is not a fund of the govt; there is no transparency about how much money has been collected, let alone spent where. I could go on about how every minority (Dalits, Muslims, tribals) have been rogered.