You are partially correct, guardians can be permanently killed through a few methods: if their ghost is destroyed they can’t be revived(though they can be healed by other ghosts given the guardian themselves still have light; or in very rare occasions can be resurrected, or gifted a second ghost), a guardians light can be drained making it impossible to be resurrected by their ghost(even guardians have their limits they can use so much of their light to the point of it not able to recover, or drained by dark magic, or even physical means such as blocking the light). This is which made sunsinger extremely powerful as they can self revive themselves even without a ghost, but this will drain their light so without a ghost to help recover that light they to will soon meet their permanent death.
English
-
Like Cayde... Or after Red Legion attack. The 2 ways Guardians can be killed.
-
Exactly these are 2 ways in which a guardian can meet their true end, thousands upon thousands veteran guardians were killed during the campaign to retake the moon from the hive, and during the Battle for Twilghit Gap. This put a real dent in the guardian population, and thus we saw a massive appearance of new guardians, such as our guardian. With so many newly born guardians it made it very difficult to train and maintain each guardian and most had to learn through trial and error which led to them relying on their almost limitless ability to be resurrected, when the light was stolen from us countless guardians were easily gunned downed. Before the Red War I believe there were about 5 million guardians spread through out the solar system. Now however that number has probably been drastically reduced, but with the traveler awakened we will see an even bigger increase in that number to prepare for the coming war with the darkness.