If you don't remove the -XX:+CMSIncrementalMode argument minecraft won't launch. Oracle had removed the -XX:+CMSIncrementalMode support so the launcher needs an update.
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Ok, this post is nearly 1 Mouth old and you din't anything. I wish you much fun after the release of java 9.
Even though Java 9 will probably be released this month, most Minecraft users will still use Java 8 as it's preset by the launcher. To use Java 9, you have to edit the launch options anyway.
I agree that this should be changed though. It just won't be a problem for unexperienced users.

See JDK 9 release notes and Bug JDK-8044022 (could be added to description).
I can only quote what I said above:
Even though Java 9 will probably be released this month, most Minecraft users will still use Java 8 as it's preset by the launcher. To use Java 9, you have to edit the launch options anyway.
I agree that this should be changed though. It just won't be a problem for unexperienced users.
Please refrain from making further comments unless you have got something important to say. The developers have stated that, to make Java 9 the default Java version for Minecraft, they need to rewrite some major code pieces (if I recall correctly, don't quote me on this), so for now, Java 8 will still be the preferred Java version.
Dupe of MCL-8255