My 20 year old Philodendron is dying and I don’t know how to save it.

This is true. I don't know why you were downvoted. From what I've read, you're supposed to allow the plant to assimilate to its new environment. Give it at least a month before repotting. Not to mention that most potting soils now already come with fertilizer and food in them, claiming to feed up to so many months etc. It's completely possible to kill a plant like this. Also, OP could have damaged the roots while repotting.

Also, OP, roots don't like light. Seeing them exposed in the full sun like this for pictures is giving me anxiety. If I had to guess, I'd also blame it on poor repotting practices and hygiene. Do what they say, and reroot it in water. Best of luck.

/r/IndoorGarden Thread Parent Link - imgur.com