Where does it say that Onchain is designed to be expensive?
If the block subsidy declines by half every four years, then how will the network be able to sustain itself other than by transaction fees?
Satoshi had nowhere said that "it's designed to be expensive", if that's what you're looking for.
I don't think it was designed to be expensive. Actually, transferring bitcoin was always very cheap (with very few moments ad exceptions). To prove that, just take a look at how many small transactions we see everyday in this forum for more than a decade.
That being said, it is very likely we will see higher fees on main chain in the future. When? Nobody knows. But it is very cheap now.
Finney predicted it, and I agree with him
I believe this will be the ultimate fate of Bitcoin, to be the "high-powered money" that serves as a reserve currency for banks that issue their own digital cash. Most Bitcoin transactions will occur between banks, to settle net transfers. Bitcoin transactions by private individuals will be as rare as... well, as Bitcoin based purchases are today.