while the DW was filling, I could have sworn I heard water running under the house
It's very possible that your dish washer drain piping is leaking. But, the drain piping is only carrying water when the dish washer is finished it's cycle and is pumping the water out; now while it's filling.
On the other hand, the water supply piping to your dish washer is under pressure 24/7, not just when the dish washer is running, so it's unlikely that a noise heard only when the dish washer starts would be due to a leak in the water supply piping.
Maybe check out your crawlspace on Friday, but I wouldn't be surprised to hear that you found nothing at all. Water flowing into your dish washer may have sounded just like water flowing into your crawlspace.
in addition.... at what point do you just simply replumb the copper pipes... I mean... If I get this fixed, it seems to me that it will just put more pressure on other weakend areas and as damp as it is under the house. I am sure that there are lots of weakened areas. Am I wrong?
You don't replumb copper pipes. Copper piping will outlast your house. To my knowledge, it is NEVER necessary to replace copper piping in a house.
Yes, you are wrong. The water pressure is the substantially same throughout the water supply piping in your house. It's only the small differences in elevation between an upstairs and downstairs plumbing fixture that results in any difference in the water pressure inside the water supply pipes. Even if you have a small leak at one spot, water can flow into the piping faster than it can leak out at that spot, so the pressure elsewhere in the piping would be substantially the same whether or not that small leak existed. So, repairing one small leak isn't going to make other joints burst.
Besides, it's rare for soldered joints in copper piping to BEGIN leaking. If they don't leak in the first few minutes they're under pressure, they typically won't for the next few centuries. But, it IS possible to bad solder joints to start leaking after a long time... it's just rare for that to happen. In my building over 22 years, it's only happened twice. And one of those times was a leak that happened in a 1 1/4 inch copper drain pipe, not a 1/2 or 3/4 inch water supply pipe, so it only leaked occasionally when water was draining through it.
And, finally, a water leak in the crawl space under your house typically won't do any damage to your house. The water leaks onto the ground or stones under there and seeps down deeper and deeper into the ground until it adds to the water table several feet to dozens of feet below the ground. You have to pay from a plumber to fix the leak, and you have to pay for the higher water bill since the leak occured, but that's about it. You just stay out of the crawl space until it dries up down there. There's no plaster damage, no wood rotting, no mold growing and no wet insulation to cause problems as a result. It's one of the more benign kinds of problems to have.