the whole point of agile is to be short term
Not really. The whole point of Agile is to iterate. This means short development cycles which include review and design rounds to adapt to changes that can and will surface throughout the project. The whole point of Agile is to eliminate problems caused by business, project, and technical goals not changing because planning is rigid and can't accommodate any changes because the process does not have room for those.
This is why this whole "things need to be planned" crowd are simply talking out of ignorance. Agile requires global planning, but on top of this supports design reviews along the way to be able to face changing needs. This requires planning in short-medium-long terms.
Don't blame Agile for your inability to plan. No one forces you not to plan ahead.
I'm inclined to interpret monorepos as an anti-pattern intended to mask away fundamental problems in the way an organization structures it's releases and dependency management.
It all boils down to being an artificial versioning constraint at the expense of autonomy and developer experience.
Huge multinationals don't have a problem in organizing all their projects as independent (and sometimes multiple) source code repositories per project. What's wrong with these small one-bus software shops that fail to do that when they operate at a scale that's orders of magnitude smaller?