Midjourney is a paid-only service anyway, you would need to enter your credit card, so no anonymity.

The issue is you have to disclose your phone number to Google, not (just) to Midjourney, AND Google will know you use Midjourney.

There's too much unnecessary connected PII data generated by such mechanisms.

There do exist anonymous credit cards, paid with cash, for fixed relatively small amounts (e.g. ~100u. In Europe there are restrictions to these kind of payment methods - cards must be below, I believe, 150€).

It is very much possible to create temporary credit cards linked to your real bank account for one-off purchases. Apple Card provides that as a service (US only) and other countries have similar systems that every bank adheres to.

Plus, you may not mind not being anonymous to Midjourney but mind not being anonymous to some other service (like Google).