To be fair, Google sends out multiple emails notifying that you won't receive new emails unless you upgrade or clear things out. If they read their emails even somewhat regularly -- which I acknowledge isn't a given for many people -- they'd know what's coming.

That isn’t the problem. The problem is Google photos pushes you to back up your tens of gigabytes of photos to your free Gmail account repeatedly until you say yes just once. At that point it fills up your email account with your photos and then disables your email until you pay them. Making statements about how often they warn you this is happening isn’t very helpful. No normal person would think of that as a consequence of using Google photos.