Just place that in an xml inside a Patches folder (rather than Defs). It's using the basic format, without checking if it actually exists, but it should require less updates than your current method, and is more compatible if other mods alter the launchers as well. More info can be found on the ludeon forums (search for xpath).

I found out the hard way when trying to fix a bug in a different mod (using a sub-mod) - redefining like you are currently didn't fix the crash, only using a patch did. The reason: the original def still existed in memory, and was still crashing things.