Yes — buy travel insurance. Not because medical care in China is expensive: it isn't, by Western standards. It is because “not expensive” is not the same as “no risk”, and the costs that actually hurt on a trip like this are the ones nobody plans for. Prepare for it rather than hope.
Why it is not optional
Care here really is affordable — that part is true, and it is not the point. A trip across the plateau involves long roads, weather, altitude and distance, and a serious problem in a remote valley is a different situation from the same problem in a city. Insurance is what keeps that a story rather than a bill.
I would not want to talk anyone out of the one piece of preparation that is entirely in your control before you leave home.
One question to ask before you buy
Ask your insurer one thing: does this policy cover me at the altitudes this trip reaches? Some policies exclude altitude above a certain height, or cover it only if you read the small print and say what you are doing. It takes two minutes at the point of purchase, and it is the difference between being covered and believing you are.
Questions I get on this one
Is it really necessary, given how cheap care is there?
What should I check for?
Does my existing policy cover me?
General advice, not a recommendation of any particular insurer. Related: altitude · safety · all essentials