Do you think that it would be possible that in the future bitcoin address may replace PGP keys?
Not all PGP users are bitcoiner and the same way not all bitcoiner are familiar with PGP. PGP came long before bitcoin and I do not think satoshi ever thought that bitcoin will be such big. So for him PGP was more sense making than singing a bitcoin address.
I am more comfortable in proving my identity using a bitcoin signed message than a PGP key. I use bitcoin addresses a lot but I barely used a PGP key. As you said it is safer, isn't it a good substitute?
I think PGP is not just for signing a message, another use case of PGP is that you can encrypt and decrypt a files.
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An interesting read ...