I've seen in multiple places project owners choosing to stick with GPLv2 to prevent the FSF from releasing a license that the project owners don't agree with
This does not prevent the FSF from releasing whatever licence they like, but it does prevent such licences from applying to the project.
Are there any real licensing dangers
No, except for the danger that the copyright owner will disagree with the new licence.
associated with this ability to seemingly change your project's license at will?
Delete the word “seemingly”: the FSF can change the project’s licence, but only under the following conditions:
- It must be done via a new version of the GPL, which would apply to all projects that chose GPLv2+ or GPLv3+, and which would become the recommended licence for most new projects.
- Affected projects must have opted into this by choosing GPLv2+ or GPLv3+, as opposed to GPLv2-only or GPLv3-only.
For example, if the FSF were to release "GPLv4" and it's just the exact text of the MIT license what would happen to projects that are licensed under GPLv3+? Could they be licensed under the terms of "GPLv4"?
Yes, that is what the ‘+’ in “GPLv3+” means.
But this only means that any additional permissions granted to the licensee in GPLv4 are available, including the right to distribute modified versions under difference licences. Any additional obligations imposed on the licensor in GPLv4 do not take effect, unless the licensor explicitly licences the project under GPLv4. The relevant text in GPLv3 section 14 is:
Later license versions may give you additional or different permissions. However, no additional obligations are imposed on any author or copyright holder as a result of your choosing to follow a later version.
GPLv2 does not have corresponding text. But the GPL FAQ says:
When you convey GPLed software, you must follow the terms and conditions of one particular version of the license. When you do so, that version defines the obligations you have. If users may also elect to use later versions of the GPL, that's merely an additional permission they have—it does not require you to fulfill the terms of the later version of the GPL as well.
Back to the question:
it's just the exact text of the MIT license
This is an interesting example, because it does not seem to be in the spirit of GPLv3, as required by GPLv3 section 14:
The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.
It seems that there is a bit of ambiguity here, as discussed in Is the “similar in spirit” provision of the GPL legally binding?.
Are there restrictions within the GPLv3 that prevent this from happening?
No, this would basically be impossible. The copyright owner is always free to licence the work under whatever other terms they like, unless they grant an exclusive licence. It seems unlikely that a public licence could be an exclusive licence, and it would definitely go against the spirit of free software licences.