The good news is that the abstraction layer is already there (as a separate Catalyst model), as the application was using an RDBMS in it's early versions, so no major restructuring is needed. I already had the "pleasure" to adapt this abstraction layer to newer MongoDB module versions (it started with v.0.700) :-) The "not an option for now" goes to replacing the whole application.

The cloud offerings are useless in this case, the application must run on premises for various reasons. Plus, I don't see how removing support for a language will push developers to use the cloud versions.