A post doc I used to work with would joke if he ever needed a backup career he would GMO tomatoes to make cocaine

The Simpsons nearly nailed it in this episode (for context, Homer accidentally cross-bred tomatoes wirh tobacco and created "tomacco" which effectively was like edible chewing tobacco). Lie and art once again imitating each other.

By the time it was banned they’d be a billionaire

OK but if ever there was a time for a PhD plant-whisperer to do a bit of a Walter White, it feels like now might be that time.

It just occurred to me that this is likely where the Tomacco plot of the Simpsons got started in the writer's room

Tomacco actually exists! A farmer and Simpsons fan grafted a tomato and a tobacco plants together, and successfully grew tomatoes containing nicotine.

The future is now!

Do they taste like grandma?

Farms that grow tomatoes don't allow smoking anywhere nearby and smokers must wash hands diligently because tobacco mosaic virus can survive all sorts of processes and still be passed on to tomato plants.