There is a very clear bias if one chain keeps the ticker, and therefore the price, so much that in practice exchanges exchanges can decide which chain "is the original one".

This. There's a bunch of waffling above about how the miners vs stakers will control stuff and so on, but in reality it's the markets which define which gets used. That's what decided the block size debate, not users spinning up nodes or miners pushing in one direction in ther other. Miners follow the market, and so will those running validators on proof of stake. Those who can influence the market have the most power here (and usually that's vocal members of the community or developers).