Executive summary: when networks get over-saturated, they become unreliable. Unreliable is bad.
Unreliable and expensive is extra bad, and that's where we're headed without an increase to the max block size.
I think I see your point of view. You see demand for on-chain transactions as a single number that grows with adoption. Once the transaction creation rate grows close to the capacity, transactions will become unreliable, and you consider this a bad thing.
And if you see Bitcoin as a payment system where guaranteed time to confirmation is a feature, I fully agree. But I think that is an unrealistic dream. It only seems reliable because of lack of use. It costs 1.5 BTC per day to create enough transactions to fill the block chain at the minimum relay fee, and a small multiple of that at actual fee levels. Assuming that rate remains similar with an increased block size, that remains cheap.
If you want transactions to be cheap, it will also be cheap to make them unreliable.