In other words, it's not gmo, but syn-bio.

(Of course, pretty much everything in the US without an organic label is an unholy mishmash of gmos, pesticides approved by captive regulators, and untold gallons of antibiotics. So probably doesn't make much of a difference anyways...)

People are eager to reduce meat consumption in general due to meat's heavy environmental impact, questionable ethics in factory farming, and general unsustainability due to a growing population. Because it's made of commonly available plant-based ingredients like pea protein and canola oil, Beyond meat is highly unlikely to have long-term health effects. Society is advanced enough to properly identify every essential nutrient that comes from meat and recreate it in a plant-based form, so why not replace meat?

Desire to avoid consumption of environmentally disastrous and morally questionable meat.

And it's not like the standard american diet has been doing a lot of favors for most people's health.

Vegetarians and vegans are an expanding market. Some of them still miss eating meat, especially comfort foods like burgers. There is a good article that talks about the research done on the heme used in Impossible Burgers. It talks about the protein, how it's produced, and how it's digested. There was even research done to see how someone with a soy allergy would react to eating heme. Obviously, it's no match for a long term study and the article mentions that but from first glance, it appears to be safe. One thing the article mentions but doesn't expand on is what is in the other 20% of the extract from the heme production.

https://newfoodeconomy.org/plant-blood-soy-leghemoglobin-imp...

you seem to be implying that long term health effects are the only or a very prominent reason for making a decision like this - why is that?

taste, environmental well being, anti-cruelty are all potential inputs into the question "should i try a meatless alternative", and i'm sure there are others

Is that how people's diet decisions work, generally? Do you think about the longterm health effects of everything you eat? Maybe but most probably don't or just latch onto the latest thing to eat/not-eat.