The weight limit counts stuff that goes into the UTXO set more than it counts the rest. It doesn't have anything to do with aggregations or discounts or whatever--  it's just the limit on what can be in a block recognizes that UTXO data takes more resources.  Neither signature data nor op-return stuff goes in the utxo.    In all fairness the up-weighing ought to be much higher but there is a tradeoff vs how much bandwidth people can cause if they make unusual transactions with more witness data than typical... and a subset of users are more bandwidth limited than memory/io/etc. limited.

Were it not for compatibility reasons scriptsigs, the txin fields, etc. would also be counted as witness data.

The weight limit counts stuff that goes into the UTXO set more than it counts the rest. It doesn't have anything to do with aggregations or discounts or whatever--  it's just the limit on what can be in a block recognizes that UTXO data takes more resources.  Neither signature data nor op-return stuff goes in the utxo.   In all fairness the up-weighing ought to be much higher but there is a tradeoff vs how much bandwidth people can cause if they make unusual transactions with more witness data than typical... and a subset of users are more bandwidth limited than memory/io/etc. limited.