On the other hand, some of the code you have to update may be for products that have been mothballed. That might end up being a waste of time if the project is

On the other hand, some of the code you have to update may be for products that have been mothballed. That might end up being a waste of time if the project is never revived. Or the code might be safety-critical and updating it is introducing unnecessary changes (and hence, unnecessary danger). Some tests may include hardware integration. For example, aircraft software may need some number of flight hours after significant changes. That's probably not going to be a part of the CI suite, and the changes will introduce a greater manual testing burden.