You can download the base game on PC for free and keep it if you get it before November 18th. They're obviously hopeful you'll upgrade to Forsaken and the upcoming DLC, but you don't need to buy anything to get the base game. https://www.pcgamer.com/destiny-2-free/
They did this for PSplus two months ago and it brought loads of people to it. Got it myself but have not played it for a while.