Years later at some moment the last miner will stop mining and bitcoin` network will stop its functioning. But I don`t think that it would happen in the next hundred years.
Range of using cryptocurrencies is too wide for actives so I can not agree with this statement. Just take a look at Ethereum, no asset can perform massive distributed computations.
Yes, I don`t think that world economic can evade this. It could be realized by choosing one cryptocurrency in which you could easily convert other currencies.
Few algorithms already exist that are not using a lot of electricity, PoS and DPoS so you can relax. In my opinion they will be mass-used in a few years
I do not think that Bitcoin will fall again to $8,000 or $9,000. Price now enough low but of course you can try to pick up at the lower price. And yes, you can buy some altcoins if you want
If you have only small amounts of coins or trade every day I think that exchange will be the best choice. If not, better keep your coins on wallets, someone can rob an exchange and you will not receive any compensation for your lost earnings.
You will definitely receive profit from holding Ethereum but already not so much as you could receive holding last year. Ethereum already has big capitalization and will rise very fast.