Disagree on the point and reasoning about the single database.
Sounds like they experienced badly managed and badly constrained database. The described fks and relations: that's what the key constraints and other guard rails and cascades are for - so that you are able to manage a schema. That's exactly how you do it: add in new tables that reference old data.
I think the regret is actually not managing the database, and not so much about having a single database.
"database is used by everyone, it becomes cared for by no one". How about "database is used by everyone, it becomes cared for by everyone".