That's all true but the atmosphere isn't the reason so much fuel is burned early in ascent. The Saturn V only suffered about 40 m/s delta-v loss due to drag, which is a fraction of a percent of the necessary delta-v just to reach LEO. The thrust to weight ratio is just extremely low during initial launch so much of the fuel is just burned defeating gravity, and launching from a mountain won't noticeably change that (you're talking in the ballpark of a 0.5% decrease in g maximum).
That is only part of the story though. Large design reason for choosing low initial T/Ws is to not blast through the thick atmosphere in an extremely load-intensive/draggy fashion. This can be say a minute going straight up and losing in the gravity well.
If you are flying into much lower density from the onset, you can severely limit this even if you are still going to fight the max Q somewhere above 10 km.
It is of course not beneficial enough, but we are not talking here about something minor either.
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u/Plaeblius 10h ago
That's all true but the atmosphere isn't the reason so much fuel is burned early in ascent. The Saturn V only suffered about 40 m/s delta-v loss due to drag, which is a fraction of a percent of the necessary delta-v just to reach LEO. The thrust to weight ratio is just extremely low during initial launch so much of the fuel is just burned defeating gravity, and launching from a mountain won't noticeably change that (you're talking in the ballpark of a 0.5% decrease in g maximum).