Yes. If altruism is required to maintain network security, the network will not remain secure.

But, people choose to mine Monero. At a loss, or with marginal profit. Why?

Theres a benefit they get from doing it that isn't directly measured in USD: they benefit from Monero existing because they get to use it. This is one hell of a benefit, one that doesnt often get quantified in these discussions. A vigilant community, voluntarily refusing to cave to tragedy of the commons, because they see how they benefit directly in their lives. You'll never get a mining farm to do it for free, but some guy running a few hashes on his PC with a node on it while he is asleep is pretty scalable. And it maintains a sense of personal ownership on the network of the participants, and not some far and away thing others worry about the details whole you "invest" in it. It's users paying for security in more than just block rewards, because they benefit from security.

So yeah, more mining profitability is needed. But not at the expense of what we currently have, we need both.