I love AD&D (played it from 1981 to 2012 as my primary edition), but I can't see how anyone can say 5e is just as complicated. I don't think it's even close.
5e has unified ability bonuses, AD&D has different bonuses and traits unique to each ability
5e has unified xp tables. AD&D did not
5e has ability checks unified with saving throws and skills. AD&D was all over the place, with not only different rules for ability checks, skills, and saving throws, but different rules for saving throws for each class.
5e uses basic ascending math. AD&D had descending AC and attack tables that didn't use linear math
5e did not have weapon vs armor tables, or heck, dozens of complicated tables that AD&D did that we all ignored anyway.
Take a peek at psionics in AD&D.
and glance at level and class limits for demihumans
While you're at it, look at the bard class.
I could really go on, but I'll spare you.