As a client, I won't actually do business with anyone who doesn't understand the need to fund those activities.
They will get swamped and fail, or the work will degrade and potentially fail as almost nothing works in a vacuum.
At a minimum, funding staying current, dealing with changes, dependent system changes, and the communication needed to make it all work will generally exceed the margins made on new projects.
If the goal is to make money and keep some of it, a retainer, or maintenance plan is needed.
Or, the expectation of paying time and materials on all post project work needs to be there, and it needs to be lean, no hassle.
My current arrangement is like this. Not quite enough to warrant a retainer, but enough that it needs to be paid for, unless I want my partner to starve or be unavailable to me when I need them.
Bundling hosting into this is a no brainer. They know that host well, you can always move, and it's efficient.