[munificent]

It's really hard to change something like pathfinding without revamping the entire game. A game's balance is like a hanging mobile. Change one aspect, and the whole thing becomes unbalanced until you adjust all of the other weights in the system.

For example, if you make the pathfinding smarter, then there's a good chance you've made all AI enemies effectively more powerful and now all of the level difficulty is too high. You may also affect the strategy of levels — levels that used to have carefully crafted chokepoints that the player must notice may no longer be chokepoints with smarter pathfinding. That in turn might lead you to want to tweak the design of the level...

[verroq]

Except this is a RTS and when players want a unit to get to a place it should get there without taking a detour across half of the map, especially when the player can visually see an optimal path.

So no, there are no downsides to better pathfinding.

[s_m_t]

consider the following scenario:

In a battle between melee and ranged units the ranged units want to maximize their surface area touching the ranged units so that all the melee units can attack at once and quickly change targets once an enemy unit has fallen. The ranged units want to minimize their surface area so that only the outer layer of the group will be attacked while every unit on the inside can freely fire on the melee units.

In an older RTS with poor pathing this meant constantly jockeying your forces around trying to either get a good surround or keeping rank. Moving your forces in certain ways could unpredictably change the formation of your units so great care had to be taken when engaging the enemy. Getting this to work out was a skill in of itself that took strategy, positioning, the ability to predict the enemies move, etc. All of this depth could take place between very small groups of very simple units.

In an RTS with perfect pathing the ranged units always move in a perfect sphere (minimizing surface area and maximizing volume) and the melee units perfectly synchronize their movements so they don't trip over each other when surrounding the enemy. Once it is decided that a group of simple melee units and ranged units will fight there really isn't much for the player to do except tell them to get on with it.

As real time strategy games have evolved along with better pathfinding, we've found out that with all things being equal, perfect pathing makes for very very boring fights. Active abilities had to be added to give the player something to do during the fight. Rock Paper Scissors type unit counters had to be added so that positioning mattered again. More AOE spells and debuffs had to be added so that another thing would force players to jockey units around.

Study the design of Starcraft BW and Starcraft II and you can see this very clearly. Certain fundamental unit matchups were completely broken by improved pathfinding such that more and more complicated mechanics had to be added to the game to give them balance and dynamism again.

Of course, this only really applies to C&C, SC, and AOE style RTS games. More modern RTS series like Men of War and Wargame don't follow this pattern because the range and lethality of the units typically far exceed their movement speeds and terrain and cover play a much larger role, but that is a whole different topic.

[AlphaSite]

It can break the whole game balance, have you ever read about Starcraft and how its wonkey pathfinding was a big part of why it was a esports success (allegedly)?