The are trillions of costumer projects outside which use VST2 versions of existing plugins.
Even if the all hosts potentially support VST3, that does not mean the transition will be flawlessly on costumers side, because of more or less subtle behavior changes.
If i cut VST2 support, in my existing plugins, this is a major incident for my customers, and finally will create a high demand of active support.
If I have parallely use two versions of JUCE, one for legacy plugins and one for newer ones, they couldn’t share the same internal codebase, because of the API changes.
I thing there is a strong demand to still support VST2, even if it can’t use all the latest technologies.
It just the task of a framework to ensure compatibility, not to remove compatibility.