What about omega-3?
Short-chain omega-3 is plentiful in flax, chia and walnuts. The long-chain forms fish are valued for originate in algae, which can be taken directly.
The omega-3s that matter most for the brain, eyes and heart are the long-chain forms EPA and DHA, and the reliable dietary source of those is oily fish. Plant foods only supply ALA, which the body converts into EPA and DHA poorly and unpredictably, so pointing to flax or walnuts does not settle the matter. A diet that cuts out fish and other animal foods therefore risks leaving people short of nutrients they genuinely need, especially over years or in pregnancy and childhood.
Plant foods supply ALA, the short-chain omega-3, in quantity. Conversion of ALA into the long-chain forms EPA and DHA is limited and varies considerably between individuals, so this is a real consideration rather than a non-issue.
The resolution is direct: fish do not synthesise EPA and DHA either. They accumulate them from algae. Algal oil supplements supply the same compounds at the original source, without the methylmercury and other contaminants that concentrate up the marine food chain. Alongside B12, this is one of the few supplements genuinely worth taking on a vegan diet.
Presenting two options as though they were the only ones available when further options exist. Also called a false dichotomy, or black-and-white thinking.
How it applies here: Treats fish as the only route to EPA and DHA, when fish are themselves obtaining these compounds from algae that people can consume directly.