It's a two-fold issue:
1. It was overly optimistic of those who designed the electrical system to have only one 10 ga wire running from the battery to supply power to the car's entire electrical system (minus starter motor power and most FI system power). And then when they selected that little four-way distribution block with spade connectors, they further tempted fate.
2. A marginal design is only going to deteriorate over 25-35-40 years of operation out in the real world of corrosion, oxidation, weather, and previous owner neglect.
Brown wire mods on later model cars usually revolve around adding additional conductor capacity to bring more power from the battery to the fuse and relay center. This could be as simple as a companion 10 ga wire directly from the battery and inserted with a 3/8" female spade connector into the unused fourth male spade in that factory 4-way connector block. Or it could be more, involving relocating the 4-way block to add additional circuits (al la Bob Brown), or involving eliminating the 4-way block entirely and doing the crimp-solder as Hussein did, or could be using a "buss bar" or distribution block concept as a central foundation for adding additional relayed circuits, as I did.