So far, SketchUp has been great at opening older files, but that may not be true forever. I have other software where this isn’t the case. Excel worksheets, for example from the 1980’s can’t be opened by today’s version. If every single old work had been opened and resaved in new formats all along over the years, they could be opened now.
This is an achievable task for computers. It could probably be done in Ruby script for that matter. Point to a folder directory and tell it to open each file and then resave in a new directory in a new or translated file format. Translating to other popular file formats (dwg, etc) would be a hedged bet, but a bet none the less on future accessibility.
Over time the media that stores your files, will give out, so you have to keep buying new media and copying everything over. I don’t see any real solutions that don’t require time, money and continual dedication to the tasks of preservation.