We are destroying ecosystems on land as well. Everything is connected and there is no free lunch — the answer isn't to completely stop doing one or the other only to greatly increase impacts on the other parts of the biosphere, it is to produce food with sustainable, science based management, which is absolutely possible in the ocean.
But that might not include anything recognisable as "fishing" or harvesting wild populations.
As was said to you eleswhere - much of the world does have a choice of where to get their calories. Taking from the sea until it has nothing left to give is a profit-driven exercise in the main, not one of caloric necessity.
> But that might not include anything recognisable as "fishing" or harvesting wild populations
That's just not true. About half of scientifically assessed fish stocks at or above target levels, thus sustainably harvested, or they are successfully rebuilding from previous overexploitation i.e. fisheries management is working. One can easily consume seafood without engaging in "Taking from the sea until it has nothing left to give". Doing so reduces the land footprint of your diet, and for some seafood like whitefish, small pelagics or cultured bivalves, you will probably decrease your carbon footprint, even relative to a vegan diet.
You're right though, people won't. People won't even stop hunting and eating known-endangered animals.