Although true in the case of many RTS games, it doesn't have to be the case. Take chess, for example. Or vCoH, for that matter. Games can continue to remain fresh and interesting as long as they have sufficient depth (by which I mean, a great number of strategic choices where no single unit/build order/tactic is objectively superior to any other in a vacuum). It is a sign that a game lacks depth when additional units are required to keep it fresh- that game is substituting depth for sheer mass.
Chess is pretty stale it is all book now, no one is reinventing the wheel in chess. and while I never played vCoh from what I understand of it, currently the strats where pretty much figured out.
When I say stale I don't mean it isn't interesting or fun I mean that strategic thought is reduced to execution. Sure the game will evolve long term left to just the way it is as certain strats become more popular, counter strats will emerge and that will happen weather relic adds any dlc or not assuming something isn't completely broken in the game.
While I don't think relic has completely done it right and needs to be a bit more transparent in some ways about how much commander dlc they are going to release and ways to purchase it in a more easier way like a commander pass.
It allows them to keep a faction with a basic core but add function on top of that in a way that changes the way faction works but by also locking you out of other possible choices. In this way each commander and unit commanders give can be independently balanced outside of the basic core. at the same time with new commanders being released as long as they are balanced properly, something the last two German commanders where not with the last dlc, it doesn't make the game pay to win but simply gives you more options.
The amount of strategic depth this game is going to have in a year is going to be crazy deep if they do it right, as a person who likes to not simply play the same old stuff over and over again I like this. now if they do it wrong we have what happened last patch. I hope that doesn't happen again.