Usually it means prolonging animal agriculture which means more harmed animals in the long run.

Plastic nets killing the environment? Let's legalize hemp everywhere and mandate hemp nets over plastic! Hemp is generally an environmental good, so you've got vegans on board, but now you've eliminated the best negative push against fishing that mainstream culture is likely to accept.

Sure,maybe you could convince them that overfishing is a problem, but now you got fish farms.

Oh hey, cow farts are destroying our atmosphere.
Carnist: "We agree. Hey look, seaweed reduces that!"

If we get them to agree that clearing forests is bad, they'll just start stacking cows in multi-floor buildings and using hydroponic grass or whatever. And now that you've got (allegedly) environmentally friendly meat, society no longer sees a problem and you're the angry radical preachy extremist vegan for wanting the harm to stop.