When my kitchen is done (assuming I bloody live long enough I've been waiting 4 years now, DFG funded...) I'll be getting the Ninja Flip alongside the Instant Pot Duo Crisp, and we won't have an oven at all!
Some things need to catch up - food packaging doesn't have timings on it for air fryers, but you can chuck in chips and some nuggets on top and do them all as one, you don't have to cook everything separately.
They cook faster.
They are a more efficient use of electricity for cooking.
They are much better at re-heating left overs than either microwave or oven.
So from those three points they are a good option for the household budget.
Will they do everything, wonderfully, and nicer than fried food - no.
I've just had tea out of mine - posh cheese on toast (cheese n onions and paprika n stuff) - I've used fancy bread so instead of toasting in the toaster ive griddled it with a drizzle of olive oil on the hob. Then dump the cheesey mix on top and into the air fryer for 5 minutes.
With the oven, id have done the same prep then would have to pre-heat our grill for 5 minutes first, THEN cook it for 5 to 8 minutes for the same results, heating a much larger space.
I don't intend on trying to use air fryers for EVERYTHING but i do think they're a better idea than heating a whole oven for one small thing.
Of course, we're cooking for just two adults, we don't cook huge multi-dish meals, roasts etc and we're not into batch baking.