Post-Crisis nostalgia. For all the talks of Silver Age nostalgia, and indeed over the years they have brought a good deal of that back too, to me, DC's collective hearts still live in the post-Crisis era. Especially with Superman.
Its not stupid at all. Its why it worked for half a century. The fallacy in your argument is the idea that them being gone destroys his link to humanity. It does not. Keeping them alive in the post-Crisis era did not create something that Superman never had before then. His love and link to humanity is something they instill upon him by raising him. So its there already. Its permanent. And its not about tragedy. Its about lessons learned in regards to what he can and can't do from an early age, and more than anything else, its a positive inspiration going forward.