There is no need to trash any message that you may want to see later.
Just create a folder & put them in there. I have some 40,000 emails in one of my Gmail accounts & nothing in my inbox. All are immediately classified & put into their respective folders. This way you lose nothing.
ANYTHING you put in trash or spam will disappear after 30 days. All other mail will be there. My mail goes back to 2004.