It may be a good idea to expose legacy system as a service, especially when the legacy context needs to be consumed by multiple new contexts. This pattern is also known as the open host pattern.

Each new context still has to translate the response from the legacy to its internals; however, with simplified open host APIs, one can mitigate the translation complexity.

With this pattern, there is a need for some modifications to the legacy context (unlike the bubble context); also, standardization of consumable API SLAs may be challenging as it has multiple consumers.

We can clearly justify that a lot of legacy systems in the real world would like to adopt DDD; however, with the lack of right patterns and given the cost and impacts, there are genuine reasons and hesitations to move toward DDD. Recognizing and harnessing these models should ease the situations and encourage organizations to adopt DDD for their legacy systems and progress toward faster delivery.