if your worry is that you want to keep opening your projects in r388, you will still be able to do that
And this is what most 'big' developers will do - stay on r388 forever.
I'm saying forever, but in fact it won't be a long term solution. This means no official support, no bug fixes, no new features. If WebView2 becomes available on Mac and Linux - we won't get it. A new version of NWJS is released (which requires updated Steam plugin) - we won't get it. Scirra adds Switch export (haha, I know) - we won't get it. Google or Apple introduce some breaking change - we won't be able to export for mobile. And so on and so on.
Another possible path where it can go - people will start using hacked versions of C3. I don't say they will necessarily stop paying the subscription. But if the choice is between abandoning the project they've spent years to develop because it uses SDK v1 addons, or using a hacked version of the software where SDK v1 is available - it's a no-brainer..