Interesting, when I was a WoW GM our special “GM WoW Client” contained a lot of features that QA would use.

One of these was showing NPC pathing in the form of dots with lines in between. At any time the WoW client would know next 5 or so waypoints of a mob that was idle patrolling.

I guess it’s more efficient to send a batch of waypoints occasionally than one at a time more regularly.

It was a interesting watching boss fights where the pathing would update quite regularly, especially if the tank had just died and it was systematically clobbering everyone else.

The algorithm is very cheap to run for a client (linear first, then diagonal to target, stitch together as many of these as it takes in the fewest number of hops), as they have full map data downloaded. For 2k concurrent players on mid range 2004 hardware (you have to remember they had kind of old school tech even for the time, though I'd love to be proven wrong on this), and with all the other interactions that needed to run before the next game cycle, I think they probably decided it was better to be clientside.

It's also possible they could have made it work, and it would have been a technical win, but just didn't need to since they had an MVP.

Validating that a path on a grid doesn't move through impassable objects is definitely much cheaper than running a pathfinding algorithm, especially on 2004 hardware. You'd have to make those checks as well in order to run the pathfinding.

It's a trade-off, especially for longer paths, especially when you count the cost of server RAM cache misses.