> why pay the huge transaction costs and help the current crypto-rich get richer, instead of making purpose-built chains for your future applications?

the most convincing arguments i’ve heard for reusing an existing chain is 1) easier access to users, 2) easier to deploy and 3) if your application needs decentralization, a mature blockchain will be more secure (attacks like 51% attacks have higher cost) and reliable (in the uptime sense) than something you can deploy yourself.

> There's vague talk about "the backbone of future banking systems" or such for crypto as having similar fundamental value

another argument is that many cryptos are valuable because of “regulatory arbitrage”. bitcoin/monero/zcash are all easy ways to (partially) shelter your money from tax and legal regulations (hence why they are/were largely associated with drugs).