The API could theoretically be replicated - its not based on proprietary data from the provider - but it would be a resource intensive effort to do so. And would involve an ongoing to hill climb to match its performance.
Ah, a tricky situation. Honestly if you can, the best choice would be to buckle down and at least replicate some functionality on your own - that way you have at least a safety net if the API is revoked or pricing goes way up