Is there anything (for example in the T&Cs) that stops Google from creeping ads to paying customers in the future?
I bet not.
I bet not.
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)