From the point of view of a customer, a complementary service would be a product that fits the same niche and is reasonably substitutable.

English is a living language so it's impossible to say definitively that you're "wrong" but I don't think anyone else is able to understand what you're trying to communicate through your use of this word.

According to the Cambridge dictionary, there are a couple of variants depending on your dialect:

- "to make something else seem better or more attractive when combining with it" - e.g. "Strawberries and cream complement each other perfectly."

- "to help make something or someone more complete or effective" - e.g. "She used photographs to complement the text of the news story. "

The only way I can make any sense of your words is to assume you're trying to argue that two competing but interchangeable services "complement" each other when used simultaneously because that might allow a customer to mitigate risk in a similar fashion to a multi-cloud setup. That's a pretty big jump to make from what you've actually said though and would really need further explanation for anyone to understand.

FYI in modern microeconomics, 'substitute' and 'complement' are precisely defined terms that have opposite meanings.

This is interesting. In university, I was taught that a substitute product would be an alternative choice (eg. Nathan's hotdogs vs Ball park franks) and complimentary products were ones that typically "moved" together (eg. Nathan's hotdogs and ketchup). So not really opposite unless you are talking about price vs demand.

No, they are in fact opposed definitions if you dig in a bit.

Substitute goods have negatively correlated demand, while complementary ones have demand that positively correlate. A simple criterion, at least in theory. You will buy more ketchup when you buy more franks, but you will buy fewer hot dogs.

And it's also close enough to the MBA definition of complement (of "commoditize your complement" fame).