The McMahon line does make sense as a de-facto border, and I wouldn't be surprised if it is eventually officially recognized by both countries. But the Simla Convention that established it was one of those Unequal Treaties, signed by the British Raj and the Republic of China - both very notably not the current governments of the two countries in question. The question is unsettled enough that they fought a war over the issue.
You might think that it's just mountainous frontier, who cares. But the Brahmaputra River runs most of it's course through China, and is heavily dammed for hydropower. It also provides most of the water for Northeast India, so you can imagine what sort of water rights disputes that leads to. So it's not quite as simple as "they just need to sit down and get it all sorted out".






from your system

What's frustrating is that even normies get it, and don't actually want this shit, but it's just being shoved down our thoughts anyhow.