You can add a column to an SQL database without difficulty. If you're using one of those "turn database rows into objects" packages, you may have problems, but if you use SQL properly, it's fine. Components that are only data consumers will not even see the new column.

There is a solution for this problem: I use a monolith that make 100% of my sql queries using LINQ (including massive updates and so). Since Linq is strongly typed you can rename/remove/split tables and columns, fix compilation issues, create a migration and deploy the new monolith. We create an average of 5 migrations per day in a project with around 300 table for 2 years and no signs of an unmantainable mud so far.

Disclaimer: I'm the main developer behind Signum Framework

Just inherited exactly that.

The final cherry is that it uses zero joins, it's all done in code.

It's going to be a bastard to unfuck that mess.