There's a couple of different issues here.

The first is that there are many what most people would call standard licenses that contain tons of restrictions. Many of them like AGPL (and GPL to a lesser extent) aren't even fully understood by lawyers.

Second, when people are talking about standard open source licenses, they are generally talking about licensees approved by the OSI. The problem is that the OSI is primarily financed by tech companies including Amazon, Microsoft, and Google. They're not ever going to add a new category of licenses or promote anything that harms those companies business interests.

As far as relicensing being a slippery slope. There's no such thing as relicensing. Once software is released under a certain license, it is always available under that license.

Anyone can choose to license future work using a different license. That is functionally equivalent to the original authors of a project ceasing work, and someone else starting work on a new fork with a different license. Nothing about Postgres using a standard license prevents this (for what it's worth Postgres is released under its own custom "PostgreSQL License" even if it is very similar to other open source licenses).

What you are actually worried about is that someone stops doing free work, which is the practical consequence of "relicensing". If that's the case you need to look into much more than the license of a project, you need to look into size of the organization, funding, size of the community, time in operation etc... I don't know if operating under a non-standard license is even in any way correlated with an organization or person relicensing or any other form of ceasing development.