That's an issue you don't have if you provide source code.
If a customer relies on bad architecture/undefined behaviour, that shouldn't pull back all the other customers that want their subscription materialize into engine improvements.
Unity doesn't have a single source code license or price. It's negotiated per-contract and usually has strings attached (e.g. be the first to ship this tech, attend UnityCon and give this demo). They only do it for big companies as well.
If a customer relies on bad architecture/undefined behaviour, that shouldn't pull back all the other customers that want their subscription materialize into engine improvements.