ASIC miner producers are billion dollar companies. They are not going anywhere and keep releasing new miners.
In terms of POW altcoins like Kadena, you shouldn't mine them 24/7. Miners are always checking the most profitable coins at the moment. So you may buy an ASIC miner for BTC but switch it to Kadena for some time (can be even a few hours) once rewards are higher so you don't need to buy expensive equipment for alts only.
I hope so. It would be up to ASIC manufacturers to make their hardware energy efficient with every release. Not only that, but miners themselves would have to rely on alternative energy sources. Once PoW becomes "environmentally-friendly", governments will have no excuse to keep targeting coins with such consensus mechanism. There's been some progress in the mining industry, so I'm confident PoW coins will be here to stay.
The more PoW coins we have, the more decentralized the industry will be. Unfortunately, PoS coins dominante a large portion of the market. Don't expect this to change anytime soon, especially when exchanges and whales are profiting from it. Money talks, right?