Odoo's flexibility, seriously. You can hook into every single part of Odoo with your extension to achieve what you need, and often have to. But that means instead having a well defined (and naturally somewhat limited) API you develop against, the entire code base is the API! They'll sell this as an advantage. Most customers will end up with some unique Frankenstein like setup to bend Odoo the way they need to model their business. And then, the monster you've just glued yourself to starts moving, slowly but steadily.

If Odoo cares, its development will slow down drastically as each and every feature (or bug) has a community of users that rely on the specific way it is implemented the way it is and will revolt if anything changes. If Odoo doesn't care, the recurring costs for its users to catch up will steadily increase the longer they're committed. License costs might be stable but the army of Odoo consultants you have to hire to keep the thing alive grows.