The solution is very scalable. 

The solution does not have any issue with scalability.

This solution is scalable, but there are always licensing cost implications. 

You can scale it, but it depends on how you scale it. If you scale it horizontally, there will be license implications.

It's an enterprise solution and we have just under 2,800 interfaces on the system. Every interface would count as a user.

IBM Sterling is quite scalable per my experience. We cover the whole sector of a broad industry and that is a lot of users. I think it is hard to say exactly how many end-users there are. But entity-wise, we are working with more than 200 companies through our instance.  

This solution is not at all scalable. 

Scalability is a bit complex.