Because of instability issues with out-dated VST2’s such as this I guess?
I mean, VST2 has reached it’s expiry in terms of support a while back, and yet there’s still another 24 months to play out. So by no means overnight switch off.
I agree the initial announcement was a little bit of a wake up call that many of us had been sleeping on, I Was initially a little upset… But really, when you process it all rationally, the active third party developers need to pull their fingers out.
For those plugins that are no longer developed, projects that contains them you should ideally be backing up to audio stems as a minimum archive requirement anyway, future projects you can just run a VST2 bridge. Or, retain an older build of Cubase. The solution is really that simple.
C12 on it’s own may happily run on whatever O/S is current in 4/5+ years, for example, so it’s not like you NEED a dongle for legacy projects either.
Developers that are still active then you need to contact them or open a ticket and make it clear you’re expecting a VST3 version to continue with their products. Also explain that if possible you want the ID to be preserved so that VST3 equivalents load in the place of VST2 - I’ve been through all of mine and sent tickets, emails, social media @'s - everything! And most have replied positively. 
But, this is has been discussed on other threads multiple times now - I think most people are just sick of discussing it, it’s happening so you need to deal with it in the ways above.
At least you’re aware of the changes and that’s the key point here is that communication seems to have pulsed out through the audio world. Those living under a rock may get caught out, perhaps.
Let’s look forward to newer MIDI 2.0 implementations with VST3, improved CPU efficiency, DAW stability and hope more developers use VST3 specific features.