Having a block size cap which would create a fee market in case of a spam attack which discourages/prevents prolonged attacks is not exactly called "network security". Security of the network relies on many things including hashrate and its decentralization.
Both of which rely on the computational power that the network incentivizes to spend, which incidentally corresponds to the cost of block space.
Not to mention that bitcoin was secure when the blocks had barely any transactions in them
Never have I argued the opposite. However, it'd be utter clueless to consider bitcoin increasing exponentially in price when the block subsidy will not be sufficient. The only realistic approach, is that transaction fees will be the backbone of the network in two decades from now. So bitcoin, more or less, relies on the fact that it will be congested.