Not quite true (well, for most advertisers; I obviously can't speak for you directly). Total return is also important. Especially when you consider that there are non-zero time costs to running advertising on yet another platform.
Would you rather make $ 1 from a $.1 investment or $ 10 k from a $ 2k investment (assuming the former isn't scalable because there are no more users to advertise to)? If there is a non trivial time cost in setting up management, billing etc, you might not even bother with the former.
Yes, in theory. In practice it can be very hard to include switching costs, complexity costs etc into the equation. Most business people I know take a much more pragmatic view of calculating ROI on direct costs and putting a finger in the air about whether it's worthwhile.
Also, if total return is small and capped, it may not even be worth the time to calculate ROI.