If you want to make your head explode, consider that solder is LEAD and if you start soldering and taping enough wires... well you get the idea. The factory used those mega splices in the dash harness which are small lead bricks.

I actually stopped soldering everything in my wiring projects. I've had it crack and cause more problems than it's worth, even when done properly. Good crimping technique and heat shrink tubing should generally be all that's needed. Most of the factory wires and connectors were just crimped and lasted 50 years if things weren't subject to wayward hands or neglect. Sometimes solder is needed in spots of course but if I can avoid it now, I do.

Last fall I replaced the entire dash harness in my truck, from the bulkhead to the fuse box and everything in between. Not a whole lot going on in that thing with creature comforts etc., so the harness is pretty bare bones. All of it together I'm guessing had to have weighed somewhere between 5-10 lbs? Not sure. But yeah, like you said, with more accessories and options it'll add up. I think guys will take out the accessories and what not but probably tend to leave the harness in.

In contrast, the wiring harnesses in modern cars dwarf what's in our '60s-'70s iron so be glad we're not dealing with it on such a huge scale.

Real-world cost aside, my next project will likely have an aftermarket harness. It's not worth the time and effort in dealing with old, corroded and bent terminals, cracked insulation and old tape adhesive. Plus, you don't add what you don't need so there's no excess baggage to deal with.