Main ending must never be bad. Bittersweet, maybe, but not bad. Imagine this:
Player finally grinded his way to the final boss. After hours of playing he got heavily invested with plot, defeated countless enemies and faced great satisfaction, because he knows: he got to the end. He will beat Big Bad and save the world!
But in the ending all his party dies.
"WHAT THE F*UCK IS THAT SH*T?!" screaming player "I WASTED TWENTY HOURS OF MY LIFE FOR THIS? F*CK YOU, AUTOR, F*CK YOU!"Allow me to show you the song on what player reeeeaaaallly wants.
So, that's my opinion on downer endings. I hate these VEEERY much.