Just a quick note to say, if you have the old free SketchUp Make 2017 on your computer (or you can find it somewhere online to download), you *can* make many things from the 3D Warehouse work.
I’ve had success downloading a desired item as a Collada file. Usually when you click download on a 3D Warehouse item, it will give you a Collada option if possible.
Once you’ve downloaded it as a zip, I double-click it to open it and save the .dae file where I want it. Then open your free Sketchup and use IMPORT to bring it in. Make any changes, then SAVE AS a .skp file.
Then you can bring it into 2020! SketchUp Make is old enough that it’s files work. I’d try bringing the item into a blank 2020 design first as a test.
In case anyone is REALLY missing the 3D Warehouse.