Perhaps you didn't understand my point.

Google could easily decide to serve ads to paying customers, and it seems likely that they've left this option open to exercise later.

The fact that they apparently "do not rely on ads" is irrelevant.

There are plenty of existent services that have revenue from customers, but still serve ads (visible examples include: pay-TV, public transport)

They could, but they would have to change their TOS, which business customers would have to agree to. Most of these customers would not, thus destroying this portion of Google's business, so they would never do that. Ads are not a part of Google's Enterprise business model. I don't know how to state this more clearly.

So what are my options if I can't use a service that can potentially serve ads? None?