Monorepo is orthogonal to services though. You can have a monorepo with multiple services in it.
Even with a monorepo, you will hit a point where you have 1, 10, 100 million lines of e.g. Python, realize you should upgrade from 3.8 to 3.14 because it's EOL, and feel a lot of pain as you have to do a big-bang, all-at-once change, fixing every single breaking change, including from libraries which you also have to update. There's no way around this in current mainstream languages.
Even with a monorepo, you will hit a point where you have 1, 10, 100 million lines of e.g. Python, realize you should upgrade from 3.8 to 3.14 because it's EOL, and feel a lot of pain as you have to do a big-bang, all-at-once change, fixing every single breaking change, including from libraries which you also have to update. There's no way around this in current mainstream languages.