To my understanding, Painters are powerful inside a Canvas, but not omnipotent. Their power is fueled by the amount of Chroma they posses. At the end, Maelle is the king of the hill of Chroma, so she holds a huge amount of it. The other family members can try to get her out, but it won't be easy and it won't be quick.
Also, I believe a Canvas world does not depend on the Painter to continue it's existence. Once a Canvas world has been made, it can continue exiting in a self-sufficient manner. The piece of Verso' soul, the Young Boy we meet in our journey, was put there by original Verso, but not as a means of sustaining the Canvas, but rather as a form of dedication to it. They took the saying "pour your soul into your work" quite literally.
When Renoir started his kill switch, he needed huge amounts of Chrome, because he needed it to overcome Verso's soul, who's continued painting worked against his erasure. Once he stops paining, the kill switch will continue it's trajectory, like a huge flood once the dam is broken, and erase everything.
I still have more to say on the ending, but I'll do it in another post. This one was mostly to address your question, with how I perceived the story,