You can totally use GPL code in a MIT-licensed project, by changing the license on the overall work to the GPL. What you can't do is, for example, use GPL code

You can totally use GPL code in a MIT-licensed project, by changing the license on the overall work to the GPL. What you can't do is, for example, use GPL code in a CDDL project, or vice versa. The Apache Foundation went through a whole long process to release the Apache License 2 when version 1 was found incompatible with the GPL. License proliferation can be a big deal. In this case it's undesirable but less of a problem. I think.