Before closing, its worth pointing out that BizTalk Services is not meant to replace BizTalk Server. While there are certainly many similarities in the capabilities offered (and more on the way), there are different reasons for using each. Here are some of the reasons you should continue using BizTalk Server on-premises:
All of your connectivity points (applications, services, and so on) are on-premises
A large investment in BizTalk Server-specific solutions—as this would be likely to require a complete rewrite, therefore, outweighing some of the benefits
Usage of capabilities that are not in BizTalk Services or will not fit the cloud model, for example, File and MQ Series adapters
Cloud is not the right solution because for example, security and/or regulatory restrictions, data classifications, or local laws may preclude sending such data over the public Internet or prevent storage of data off-premises