Trying to install ezpublish-legacy via composer, version 2013.09, only 3 extensions are installed:
ezformtoken
ezjscore
ezoe

In contrast, an ezp EE 5.1 install also includes:
ezautosave
ezie
ezmbpaex
ezmultiupload
ezodf
ezprestapiprovider
ezscriptmonitor
ezsi

If these are not deemed "necessary", it would be quite easy to declare them as part of another composer-package, of type metapackage (that is, a package only including more dependencies) which users could easily add.
The package could be named f.e. ezsystems/ezpublish-legacy-default-extensions-bundle