I have a few thoughts:
First, a rust-related blowout is likely to be a point leak, first - it might be startling and disruptive, but most likely (not <I>certainly</I>) not explosive. I'd be a lot more concerned about an over-pressure rupture, which will be explosive. I recently noticed that my 6-gal, 150 psi pancake compressor was still running at about 180 psi - long after the cutoff should have stopped it, and after the overpressure valve should have released. Both were not functioning. Replacing the overpressure valve is cheaper than replacing me.
Second, you absolutely should empty it to zero psi every time you use it. If the valve is sticky, sharp, or awkward, jury rig something that you can use without cutting your fingers, something you can get leverage on, or make sure you use just a hint of anti-seize compound on the threads so it doesn't rust shut but still seals. (That pancake compressor, a Porter-Cable, is a finger-slicer. :( ) Make sure the valve is down so it drains, I know you know that, but I had to show my kid once - he had to tip the compressor to get a good grip on the poorly-designed valve and he didn't tip it back!
Third, you've had a good life with that compressor, and (as my grandpa used to say) "it don't owe you a dime." If it really concerns you, you can get into a new comparable model for right around $200 on the low end, all the way up to $450 or so on the DeWalt end. I have a 30-gal/175 psi Husky vertical that's performed very well, and its 20-gal/200 psi little brother is going for just over $200 at your local Home Despot. Small price to pay to get rid of that nervous feeling every time you turn your back on the compressor.
They aren't meant to last forever - at a certain point, we put them out to pasture, I guess, and get new ones. And I hope it goes without saying, that's something I would never buy used.