So a couple things. Its never actually said whether Jaren’s Ghost chose Shin as a new Guardian, or if Shin just had an overwhelming amount of light contained within that he was able to manifest it like a Guardian would. Why is this important? Well, this would mean that Shin is actually a Mortal, and one of the few if not the only Mortal capable of wielding the Light, which could play a very interesting dichotomy off of Bael, being that Bael is only a mortal, but is only capable of using Darkness. There is also reason why we should think of Shin as a mortal over a Lightbearer, in spite of being able to use the Light. If a Ghost is able to choose a new Risen, then by all means, those who’ve had their Ghosts destroyed should be capable of gaining new Ghosts. Yet characters like Zavala and Osiris, very very important and notable figures, never see a Ghost come to choose them. There is also the factor of Fynch having not found a new Guardian in spite of vehemently denying the resurrection of his Hive Knight. Lastly, there was a deleted cutscene at the end of Final Shape where instead of Cayde resurrecting Ghost, the Guardian is the one to do so, using both darkness and LIGHT together to resurrect Ghost. While this is no longer canon, having not taken place within the story, it does show where Bungies narrative mindset is when it comes to these types of situations. It’s not that Guardians are incapable of using the Light after their Ghosts die, rather it might simply be the case that very few and very special Guardians like that of Shin Malphur and Young Wolf/Hero of the Red War/The Guardian are the only ones capable of such feats. All in All, there is more to suggest that a Guardian cannot be rechosen, and rather, if they have an overwhelming wellspring of Light, like Shin and the Guardian, they are capable of using the Light like a Guardian after they lose their Ghost.