The reason I think this is that every new industry is going to have inefficiencies in it and like anything, the unit cost of production for the first units are astronomically expensive, until innovation advances the processes and tools and economies of scale are reached. For example, how much electricity does it take to mine HIVE? Well, that requires the nodes that process the transactions and the people using their computers and phones to interact with it, which because it is done on the surface at the second-layer, is effectively the same as the cost of surfing the internet.
No. The massive energy use is the whole point.
Proof-of-Work deliberately uses a great deal of energy because the sink cost of mining prevents a 51% attack. What means is that if there was no cost to mining a block, then in such a permissionless blockchain such as Bitcoin, a single entity could attack the network by signing a very large number of blocks with falsified information to undermine consensus. Because a proof of work is required, a 51% attack on Bitcoin would be prohibitively expensive.
Proof-of-Stake chains require acquired stake from block validators. Delegated-Proof-of-Stake requires delegated stake from block validators, which is essentially permission granted by stakeholders.
Bitcoin's value proposition is high security through high cost of mining.
Efficiencies have nothing to do with it. Your colleagues asked whether paying the price for Bitcoin's security is worth it. The answer depends on how much you believe in authority. I don't trust monetary authorities and governments not to fuck up the economy eventually. (See @taskmaster4450's posts about the corner the Fed has painted itself in a corner already in the 1990's by printing money excessively and creating a perpetual low interest rate environment leading to the over-financialization of the economy, zombie corporations and so on and so forth.)
I think you have misunderstood, as this is about the entire industry. Also, the mass of power isn't necessary for security, although that is the case now in regards to bitcoin.
Also, it is possible that the value of bitcoin already mined and being used is more attractive than mining the remaining bitcoin, where what is left in the ground just isn't worth it. Transactions still need to be signed though.