Too Long; Didn't Read
There are over 24 layer-1 blockchain projects just within the first 100 listings in coinmarketcap.com. Each of them claims to be the “next-big-thing”, still they are just a broken iteration of one other. One of the biggest issues is not being able to scale without trading off either security or decentralization. Sharding your system into millions of partitions, and making sure no transactions cross any of these partitions, is the ultimate constraint on scalability. The limiting factor ends up not being processing but I/O, just because of how we organize the state in the blockchain.