Well it takes 3 crew to man an At At. So there's not much manpower they're wasting. Second its probably a good escort walker and a great siege platform regardless of any weaknesses. There is also another instance in new canon where old tech beats New. Leia and the Queen of Naboo take old N1 star fighters and take them to destroy the sats that are virtually destroying Naboo by changing its climate.
Fair enough I will take that as a example but still wouldn't they still be outdated? Unless their armor is upgraded, resistance ground forces would crush them easily.
Also I don't really remember much from Shattered Empire except for the Darth Maul thing Leia has. I generally hate.... with a passion a one year easy wipe out of the Empire. A thirty year gap should be use to its full potential, buts it isn't. You could have the Empire offically surrender 5 years later(with some Imperial Warlords making fortress worlds) and I would honestly be fine with that. Hell go one step further, and say members of Imperial High Command Fled into the Unknown Regions, and have them slowly round up small Imperial fleets to join them. That I would actually believe is more realistic, and also allows for new stories to be told and give the Empire a great fall.
Star wars dream movie: Battle of Jakku with Kit Harington playing a Imperial Captain that Commands a Raider Corvette called LongClaw
(Nice Game of thrones reference)