I just want Sketchup 15 back!

Not true! There is no reason (and nothing says that) you must uninstall the previous (or any previous) SketchUp version, when “roadtesting” any new version.

All SketchUp versions install into there OWN versioned program folder, and use their OWN settings and plugins folders. So one version install, will not (drastically) interfere with another.

There are a few quirks that can be dealt with, such as each version keeps setting itself to be the double-click opener application for .SKP files each time they run, and v2017 sets the IE WebDialog browser emulation setting for all versions of “sketchup.exe” to MSIE v11. (Regardless of what YOU use to browse the web, SketchUp extensions and some native network features use a MSIE library to display interactive content in browser frames.)

I’d suggest you at least try to upgrade to SketchUp Make 2016. There are links in this post:

Otherwise here’s links to SketchUp Make 2015: