The way hot water heaters are designed the incoming cold water feeds to the bottom, hot is drawn off the top. This takes into account warmer water rising, and cooler water sinking, it's a more efficient design, one that has been used for decades. Also, copper is very expensive compared to lined steel, which most all newer residential water heaters are made of. I could imagine the scrap value on an old copper heater, that would be a find!
There's a reason for that drain on the bottom of water heaters, other than to drain when replacing. Maintenance on a hot water heater includes draining from the bottom until it runs clear twice yearly. Besides removing this debris from the system, it keeps the heater running more efficiently, as you are heating water when it's clean, not debris.
The debris in the bottom of water heaters is partially from the pipes, which will at times build up lime scale or corrode a bit. The majority is from an anode in the tank, which is designed to corrode rather than the water heater itself doing so. This increases heater life as well, if the tank or connecting pipes rust out the heater needs replacing.
You'll get the same pipe debris in cold or hot supplies, that's why you remove faucet aerators after having shut the system down & run them for a minute. The debris will clog the aerator, reducing flow at the faucet.
No, I'm not a plumber or some such, just on my third house in 3 decades, and have replaced countless water heaters, among other things.
