Some things to point out:
Existing models can still be downloaded in the formats that are automatically created, and in the format they were uploaded as. That should mean that most of the over 4 million models that are there already should continue to be available for 2017 users.
Eventually there will be a point where you are looking for a model in the 3D Warehouse window, or in a browser, and now you find out that it’s a recent model that doesn’t have a 2017 conversion. You can get around that issue by searching in the Components panel. Then you will only be shown models that are compatible with your version.
If you like to upload models for others to use, and you want to make sure that 2017 users can still use them, you could Save As to 2017 before uploading the model.
I am not sure of the future state of Collada, but assuming that sticks around you can download in that format, and even load it into very old versions of SketchUp.