Well it is possible to rollback. Download the Mojave application from Apple Support to Applications. Through your store account. Take a USB stick and erase the content. name it. See the command for the terminal.
Change the name of your USBstick in the command to the real name.
Open the terminal and the paste this command in your terminal. After several minutes you have an install stick with the OS you want to use on your MBP 2017
sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/yourUSBstick && echo Mojave stick made; say Mojave stick made