There is a such thing as too much trial and error, especially in the days before the internet. And the Zelda I overworld is exactly that. The dungeon design was top notch, though.
Most of the "difficulty" in Zelda I can simply be attributed to not being able to move diagonally, and generally less polish in the controls. Which is fine for the time, but would be stupid to keep around. I agree that the games could (should) be more difficult, but don't use Zelda I as the benchmark for how to do that.
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"One toot on this whistle will take you to a far away land."
-Toad, SMB3