No. As a general rule with all software, you purchase a license to use the software, not the actual software itself. That being said, GOG and Itch.io can't yank games that you've already downloaded. I don't know if Steam does or not, but it probably can.
The Content and Services are licensed, not sold. Your license confers no title or ownership in the Content and Services. To make use of the Content and Services, you must have a Steam Account and you may be required to be running the Steam client and maintaining a connection to the Internet.
But steam some track record for keeping game. Game removed from store (like rocket league) still in library.
Yes but if Valve goes bankrupt (unlikely today but the winds may change), imagine the amount of backlash caused by people not being able to download "their" games since the servers would have been sold.