Many vegetarians don't eat animals because they feel pain not because they are alive, were that the case they would eat animals that died of old age. Plants have no mechanism to feel pain and some plants specifically spread through consumption, though there are some plants that vegans won't eat because insects play a necessary part in their creation i.e. almonds if memory serves correct, they don't eat honey for the same reason. Vegans generally want no animal or insect, sometimes even bacteria involved in the creation of there food and there's numerous n different forms of veganism that in not knowledgeable enough to describe. There's debate about some sea life for vegetarians, however recent information highly suggest insects feel pain and most vegetarians probably wouldn't include intentional insect consumption as valid vegetarianism (there's no real avoiding unintentionally consuming some). It's not grossness thing. Most people who are vegetarian started as meat eaters and chose for a time or for life they were no longer comfortable with the equation of enjoyment of eating meat versus the harm to other creatures they were causing. Everyone needs to figure out their own line of comfort and that changes over a lifetime.

Your arguments about sustainability while not incorrect don't actually factor into the debate about whether or not insects are agreeable to a vegetarian diet and lifestyle. They are more of a concern about animal welfare which doesn't require a vegetarian diet. There are numerous animal advocates and farmers who advocate for animal welfare without practicing a diet other than omnivorous.