Even in Oracle, we've had to schedule downtime for a single column schema change and population for 45 minutes. I would call Oracle "mature."

Note: I wasn't involved in doing the update, so it's possible there was a better way to do it. The table is used by about 50 different applications.

True in this case, but not always true, the issue often isn't upgrading the schema, it's that there can be no single schema that can hold the data because it varies on a per row basis. No RDBMS can deal with this well, no matter how mature, it's just not what they're designed for. This is trivial for an OODBMS which just stores raw objects, no matter their shape.