That only works if Thoughbot's open source tools are good enough. People seem very happy to pay for JetBrain's suite of tools. Postman's collaborative offering makes good money. Sublime is still doing well.
There are people still selling standalone pieces of software to developers and doing okay. It's hard to compete with huge companies with a build-over-buy culture but that's just their nature. It's way cheaper to just built it themselves and have control of the project than to pay the licensing fees.
Tools are an easier sell. You can always switch to a different one without too much fuss. That's a lot different than writing five years worth of test cases and then having to go for a license renewal.
There are people still selling standalone pieces of software to developers and doing okay. It's hard to compete with huge companies with a build-over-buy culture but that's just their nature. It's way cheaper to just built it themselves and have control of the project than to pay the licensing fees.