Here is a non-exhaustive list of possible reasons: - you use in-memory cache, so with monolith each instance will need X+Y amount of ram, where X is needed for

Here is a non-exhaustive list of possible reasons: - you use in-memory cache, so with monolith each instance will need X+Y amount of ram, where X is needed for api A and Y is needed for api B. - say api A serves client http requests, and api B does some asynchronous resource intensive computations. When a large workload lands on api B, api A will suffer until auto-scaling kicks in. Even after auto-scaling kicks in, individual nodes could be overloaded which is fine for api B but unacceptable for api A. - in a scenario where api A and B both serve client traffic, maybe you want to switch api A load balancing from round-robin to sticky, now you have to worry about the impact on api B as well. The possible scenarios are endless, and you always have to think about both api.