I disagree about Light, the problem is that it only matters to Dark. How can you type it to anything else? What does light do other than counter darkness? Not normal, not fire, not water, not grass, not bug, not psychic, maybe ice, not poison, not dragon, etc. They should have went went something other than pink though for the colour.
Not true.
Just like Fairy wouldn't be just girly fantasy tropes, Light wouldn't be just religious fantasy tropes.
Light
can mean things like angels, but light is also what the the sun emits, light is what lasers are made of, insects and fish are often attracted to light, plants require light to photosynthesize, many reptiles need access to UVB light for proper bone growth, light is generated by some chemical reactions....
I could probably go on for a whole
page, and most of those things could be made into Pokemon concepts or game mechanics for Light.
Hence why some old mons get retyping.
So?
Fairies are usually about mysticism, magic, spirituality, and mischief, but all of those themes are already conveyed by Psychic, Ghost, and perhaps Dark. So the developers decided to do the re-typing.
Light would have some theme overlap between Electric and Grass, but that's probably all. Fire produces photons, but it also produces a lot of heat, and doesn't have any other themes in common. Re-typing would still happen, but it would be more meaningful, because of how important light is in the universe and because of how easy it would be to use thematically (outside of religious stuff, which is usually the way it ends up).
Some examples:
Dark/Light - Star/Nova pokemon
Bug/Light - Retyping for Illumise/Volbeat
Grass/Light - Retyping for Sunflora
Water/Light - Anglerfish pokemon (more realistic than Lanturn)
Rock/Light - Crystal pokemon
Dragon/Light - New Dragon type, or maybe a Dunsparce evolution.
Steel/Light - Knight pokemon
Ghost/Light - Invisibility pokemon (perhaps a reference to The Invisible Man classic sci fi movie)
Light could also be combined with pretty much any animal for a bioluminescent version of that animal.