> They've essentially gotten roped into maintaining a huge chunk of internet infrastructure, for free. If they ever shut it down the whole world would end up rioting because it's so widely used.

You have a point, but if you've ever seen how a gmail account behaves for the ordinary person once it reaches 80-90% storage capacity used (15GB free, some cumulative total of all emails and google drive content, google photos content), all of these free services exist to sell a perpetual monthly recurring subscription to users. And many people do pay. The default gmail web interface starts to have a big banner across the top warning about storage reaching maximum capacity with a link to the payment page.

Look at the workflow for a standard out of box android phone now that defaults to backing up all your photos to 'the cloud', which will almost immediately fill the 15GB free. Once your 15GB is full, then you're run through the payment/checkout workflow to enter your card and set up monthly recurring billing for some premium google service.

In general having a gmail account is the initial stage in the pipeline of getting someone to be a monthly-paid google customer for life. Whether it's just for more storage to hold all their google drive and photo content, or google workspace individual, etc.

Additionally, tying a gmail account to the primary-user android on-device account on any android 4.x+ device means revenue from google play store paid app sales. And then all those 'free' apps that the user installs where the app developer has implemented embedded small ad banners for google's ad network? More venue.