> You can't just make up your own domain and hope no one ever does anything conflicting with it.
Actually you can! You just need to use one reserved for that purpose:
> In 1999, the Internet Engineering Task Force reserved the DNS labels example, invalid, localhost, and test so that they may not be installed into the root zone of the Domain Name System.
Sure, but you should NEVER use a domain that isn’t delegated to you or explicitly reserved by the IANA for local purposes. Plenty of people used .int as well which is now a gTLD, you can’t complain about a name you don’t own breaking down the line.
Actually you can! You just need to use one reserved for that purpose:
> In 1999, the Internet Engineering Task Force reserved the DNS labels example, invalid, localhost, and test so that they may not be installed into the root zone of the Domain Name System.
https://en.wikipedia.org/wiki/.test