Agreed. But if a user agrees to be left "out in the cold", you should allow him to. Given that it is not very costly for you to prepare your software for on-prem, it maybe a win-win for all the parties involved.
I have been asserting exactly the opposite in this thread.
None of which are insignificant.
In addition to that you must now support (and test for) a million configurations, rather than just one.
I also have experience developing and maintaining cloud solutions. Total amount of work that goes into large cloud apps and amount of things things that can brake or not work properly is fairly impressive. Definitely not any less then on premises