There are two ways to try to scale a blockchain: fundamental technical improvements, and simply increasing the parameters. Increasing the parameters sounds very attractive at first: if you do the math on a napkin, it is easy to convince yourself that a consumer laptop can process thousands of transactions per second, no ZK-SNARKs or rollups or sharding required. Unfortunately, there are many subtle reasons why this approach is fundamentally flawed.
Computers running blockchain nodes cannot spend 100% of CPU power validating the chain; they need a large safety margin to resist unexpected DoS attacks, they need spare capacity for tasks like processing transactions in the mempool, and you don’t want running a node on a computer to make that computer unusable for any other applications at the same time. Bandwidth similarly has overhead: a 10 MB/s connection does NOT mean you can have a 10 megabyte block every second! A 1-5 megabyte block every 12 seconds, maybe. And it is the same with storage. Increasing hardware requirements for running a node and limiting node-running to specialized actors is not a solution. For a blockchain to be decentralized, it’s crucially important for regular users to be able to run a node, and to have a culture where running nodes is a common activity.
Fundamental technical improvements, on the other hand, can work. Currently, the main bottleneck in Ethereum is storage size, and statelessness and state expiry can fix this and allow an increase of perhaps up to ~3x – but not more, as we want running a node to become easier than it is today. Sharded blockchains can scale much further, because no single node in a sharded blockchain needs to process every transaction. But even there, there are limits to capacity: as capacity goes up, the minimum safe user count goes up, and the cost of archiving the chain (and the risk that data is lost if no one bothers to archive the chain) goes up. But we don’t have to worry too much: those limits are high enough that we can probably process over a million transactions per second with the full security of a blockchain. But it’s going to take work to do this without sacrificing the decentralization that makes blockchains so valuable.