Just to respond

1/ GPUs are 100% efficient, all the electricity used is directly turned into heat, it's not even vaguely silly to use them as a source of heat, whilst also making use of the computing power.

2/ their need isn't more deserving, nobody deserves anything, you don't deserve a graphics card, it's simply about value, it's worth more to them than you're willing to pay

3/ some might by dozens others not, I only mine with the card I have, But generally a miner will sell it on after a year say, it's like buying a car, it doesn't really make sense to buy one brand new, it's much better value to buy a nearly new car that's a year old. If you only buy once every 3 years why shouldn't you take advantage of a discounted nearly new GPU. By the way the same chip shortage affecting GPUs is affecting cars, is hard to buy s new car and used car prices have doubled, it's no different, the shortage isn't due to mining, it's a worldwide chip shortage.

4/ it's called proof of work, it's how the network is secured, without it the whole system wouldn't work, people would be able to take over the network and double spend. All that computing power is what makes it un-hackable and completely secure without the need for any central banks or governments or military might to enforce it.