Tendermint works great for 4 to 300 validators, and with the Cosmos delegation model, many more stakers can participate. So yes, it's not a trivial problem, but Tendermint is solving it.
Tendermint is not solving the problem I pointed before because it is not trustless, you are trusting the validators.
In PoW, you are trusting the miners.
I am not talking about PoW specifically but about the new research that Bitcoin opened, including new consensus systems in trustless networks. The community knew how to build distributed systems in trusted networks before Bitcoin.
Yes, there were many designs.
I don't know of anyone who really applied traditional BFT networks into public blockchain architecture, however, until Tendermint.
For example, who spoke or wrote of BFT fault accountability/attribution, and how it pertains to proof-of-stake models, before Tendermint?
There was also the 2011 paper that lamented the lack of good open-source middleware for BFT algos. Tendermint solves that engineering problem as well.
Regarding private blockchains growing to public trustless blockchains, that is not a trivial problem. There are new blockchain models that are being promoted (e.g. IOTA, EOS, NEM) which have not being studied enough by consensus researchers.