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JUnit4Migration
The text below is one user's personal opinion. JDT will not add support for JUnit 5 if that would mean dropping support for JUnit 3 tests, and Eclipse projects are currently not required to waste time converting their tests. Converting tests is not trivial, and any contributions need to be verified carefully. See e.g. all the trouble we had in bug 487095. Green tests doesn't mean the conversion was done properly.
With JUnit 5 alpha being already available (http://repo1.maven.org/maven2/org/junit/) it seems like it will drop support for JUnit 3 APIs. Once JUnit 5 comes to JDT it would be nice not to have 3 different style of junit tests around. This work will have to eventually be done to keep up with the newer JUnit releases.
Also https://junit-team.github.io/junit5/#running-tests does not even mention the possibility of running JUnit 3 tests.
The APIs are cleaner in most cases and most developers are more used to 4 API rather than 3, and that is always good for the project in terms of potential contributors.
To summarize the goal is: make sure for all (as much as we can) projects to be prepared when JUnit 5 comes and it kills support for JUnit 3 tests.