They would if they were all cooked in the same sized cavity, but a Ninja, for example, will make a casserole in a pot-sized vessel, which is all that needs heating up, whereas you would have to heat the whole of the oven, which will use more fuel to bring to the same temperature (unless I'm missing something, which is perfectly possible
).
Similarly a slow cooker used for rice pudding will just heat the pot that it's served in, so again, you don't need to have the whole oven on.
What I am saying is that where the oven comes into its own is that you can cook them both at the same time and maybe a tray of scones as well, so much of whether someone would want to get rid of the oven (for financial reasons, at least) depends on how often they cook with it full.
I can't imagine getting rid of mine any time soon, but I do love kitchen gadgets, and use the majority of the ones I have (the spiraliser being the most useless, so far).