Regardless of altitude, most individuals do not drink enough fluids, with active individuals it is even worse. Drinking the right balance of electrolytes is more important than drinking lots. Last thing you want to do is de-electrolyte yourself by flushing them out of your body. While rare it does happen and is extremely dangerous.
I'm gonna disagree with you here. There is a modern group of thought that the mass public is not really chronically dehydrated and the practical advice is to drink when thirsty. Unfortunately people are not practical nor do they always listen to their bodies so this isn't the advice that is always put out.
I would counter that the real known problem at least, in the US though is drinking either empty calories from beverages including from electrolyte/hydration "sports" drinks, or artificial sweetened beverages. Whereas most individual body health/hydration would be better off drinking just plain water. There are lots of folks who don't drink plain water at all and live their lives only drinking Diet Coke or flavored sparkling water.
So there is a two pronged push in this underhydration myth in this fight over your liquid intake.
a) From Big Drink. Obviously the beverage industry is trying to sell you more beverage products
b) On the Medical side the advice to drink water is not really about dehydration but a nudge technique to push you to reduce drinks and replace with water. The key here is wanted to use a nonnag technique and telling people what you should do, rather than saying what they shouldn't do. There is also a strategic nuance of a preemptive strike before there is a conscious decision about being thirsty and choosing between water and a drink.
This is "in general" advice for long term steady-state average (mostly sedentary) US citizen that you are bringing up. For new or unique situations like this OP scenario of going skiing at altitude (assume for the first time); or about to run a marathon for the first time or some strenuous thing the body is not used to; of course you should take some special measures and special advice to prepare for that new scenario your body is not accustomed to.
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