Correct. If you didn't mind having a Mojang account (small, indie, Swedish company; respectable privacy policy) but would prefer not to create a Microsoft account, your money is as good as gone and there is no way to play the game officially.
if you just want to play offline, the prism launcher still lets you authenticate using the old mojang accounts. (which is not to contradict your claim, as it's not an official launcher).
I held out for a really long time, but a friend wanted to play minecraft with me so I finally caved. If they ask me to add a phone number I'll probably just abandon the account though, and look into piracy of the game I purchased.
After converting, my brand new MS account got locked for 'suspicious activity' within minutes. To get back access to the game I had paid money for, I had to... you guessed it: add my phone number.
Really makes me consider just not playing anymore.
