I'm not so sure that altcoins have a lot more throughput than Bitcoin after reading this summary of testnet limitations of BitShare 2.0, which is claiming it an reach 100k TPS in the real-world:
Bitshares full nodes are very different to Bitcoin. Bitshares is delegated proof of stake, as far as I understand it there are only 100 full nodes which are incentivized and voted into position by the users based on the amount they hold. Other examples would be Dash which has fully incentivized full nodes implemented in a more decentralized fashion compared to Bitcoin. Ethereum also has some interesting solutions to scalability as well.
Yes, and that is sort of my point. You can throw out PoW, relieving a lot of CPU/GPU/ASIC intensive work (without getting into security implications), and, like Bitcoin,
the primary bottlenck is still networking.