For interaction? Pseudonyms with a ramp up into being able to interact fully is the middle ground. Your activity on that specific site will be monitored to kick you out if you behave inappropriately, but it shouldn't carry across sites unless you voluntarily use a third party identity provider (which is a good option to have).

Massive scale is a big part of the issue. It raises the barrier to entry for competing platforms (because being able to scale to rapid growth is a huge up front investment, and can easily cripple your platform if you don't do so), and brings the moderation responsibilities beyond anything actually manageable. Small to mid sized communities being the norm is much more manageable, much easier to develop for, and much healthier generally.