Implementing microservices creates additional complexity at the start; all the microservices need their own infrastructure, and at the same time, the Monolith has to be supported.
The microservices require a lot more servers and therefore pose very different challenges. Monitoring and log file processing has to deal with the fact that data originates from different servers. As a result, information has to be centrally consolidated. A substantially larger number of servers must be handled, not only in production, but also in the different test stages and team environments. This is only possible with good infrastructure automation. It is necessary to support different types of infrastructure for the monolith and the microservices, and this leads to substantially more servers overall.
Entire Migration Lengthy
The added complexity due to the two different software types will persist for a long time, as it is a very lengthy process to completely migrate away from the monolith. If the monolith is never entirely replaced, the additional infrastructure costs will remain as well.