You don't need to start a brand new chain. Hardforking implies that you'll have the exact same chain, until date X, when your rules will start being enforced. Bitcoin Cash is an example.
I know you can do that but the problem of low hashrate and possible 51% attack still remains in this scenario.
A scalable blockchain is one where payments occur in second layers, not in the main layer. If you think that by simply rising the block size, you can solve the scalability problem, then you'll have to explain to me why Bitcoin Cash and numerous of other altcoins were such terrible failures.
It's about removing bottlenecks from the protocol and pushing scalability problem to the hardware layer.
When Bitcoin network was recently spammed even LN had trouble to work properly.