Why didn't we switch to 4e/5e?

[Psyren]

Admittedly, I've only played about 4 sessions of 5E, and I suspect the DM is using the "Did you roll low/middle/high?" Task Resolution System (i.e., the "Eyeball" method) rather than actually using whatever DC might be in the book.

Gods above, why have a skills system at all then?

Well, that's not exactly correct. The DMG just says that a CR 9 creature is supposed to be a "good challenge" for a group of 4 9th level characters. So, if "good challenge" is 100% chance of success for you, then a Fighter 9 should have a 25% chance of success, but if "good challenge" is less that 100% chance of success, then that chance of any single character succeeding is going to be lower as well.

The DMG doesn't really outline challenges for individual characters, but for parties.

This, and also - CR/APL includes WBL, so "go build a fighter without magic items" makes no sense.