[cies]

Dual licensing also makes it IMHO less likely that a project "continues as proprietary". Example: Qt.

I think "contributor agreements" are the biggest red flag. Though I like them for potentially upgrading a license (say from GPLv2 to v3), not that this always is a good thing.

[aragilar]

It's also worth mentioning the specific agreement between KDE and Qt (https://kde.org/community/whatiskde/kdefreeqtfoundation/ and https://www.qt.io/faq/3.2.-why-do-you-have-an-agreement-with...), which shifts the incentives as well.

[pxc]

You don't need contributor license agreements for upgrading to future versions of the GPL. You can just license the code under 'GPL version N or later': https://www.gnu.org/licenses/license-compatibility.html