> Everything should be a drop-in replacement
This is not true for many applications. Due to the removal of many APIs from the JDK with Java 9, I needed the following dependency artifactIds to be able to move a JEE application with SOAP web services to Java 11: jaxb-api, jaxb-core, jaxb-runtime, istack-commons-runtime, jboss-jaxws-api_2.2_spec, glassfish-corba-omgapi, jboss-annotations-api_1.2_spec, activation, jboss-saaj-api_1.3_spec, saaj-impl, stax-ex, jsr181-api, txw2.
Many of these spec API/implementations are provided by different artifacts that are incompatible with each other. Some I only discovered when something failed at runtime as they perform implementation lookups and you don't get compile errors.
Additionally, many of the Maven plugins we used no longer worked and our application server failed to start.
This is not true for many applications. Due to the removal of many APIs from the JDK with Java 9, I needed the following dependency artifactIds to be able to move a JEE application with SOAP web services to Java 11: jaxb-api, jaxb-core, jaxb-runtime, istack-commons-runtime, jboss-jaxws-api_2.2_spec, glassfish-corba-omgapi, jboss-annotations-api_1.2_spec, activation, jboss-saaj-api_1.3_spec, saaj-impl, stax-ex, jsr181-api, txw2.
Many of these spec API/implementations are provided by different artifacts that are incompatible with each other. Some I only discovered when something failed at runtime as they perform implementation lookups and you don't get compile errors.
Additionally, many of the Maven plugins we used no longer worked and our application server failed to start.