Porting games to Linux is definitely not something you do for the money. Linux sales on Steam hover in the 1%-2% area of total sales, less than even MacOS. On the plus side, if you use an engine like Unity, making a binary for Linux is no longer much of a problem (you don't have to wrangle xplatform libraries etc). On the minus side, actually testing and debugging things to make sure things work for customers who use…
Part of the article suggested that building Linux first is easier, because porting to Windows should mostly just work.