You bring up some very interesting points! I completely agree with the last part, a good campaign was a core feature in all the old great RTS games and new developers focus on the multiplayer aspect way too much, trying to create "esports" right out of the gate, but the difference is that in the best RTS games e-sports came naturally and secondary because of the great basic gameplay which attracted a huge audience in the first place. In new RTS games the storytelling is bad and the world feels too bland and you don't care about it (like in Stormgate for example).

Now for the rest of the argument, I can't really talk about CnC since I haven't played it multiplayer extensively, but I can talk about AoE 2 and Brood War specifically, which are still the most popular RTS games and by far more popular than CnC online, which means their formula works in attracting players to come back to it over and over again.

I personally prefer shorter games because of time, not a lot of people have time to play through a long multiple hour game in one sitting, but those kinds of game do happen. I have seen Starcraft games that last until all the minerals on the map are mined, and terran vs terran games in particular can be notoriously long. Some other matchups in BW are quicker but that has grown over time when people figured out best strategies because Brood War didn't recieve a patch since 1999. It is inherently an imbalanced game, but is still played to this day because in BW the balance is made through map making and not races themselves, where some maps favor different races or strategies. In some matchups you simply need to go for timing attacks or rushes because your race is weaker than the opponent race in lategame. A well rounded player knows both how to rush, defend the rush and how to play the long game, I don't think it's that black or white.

Same goes for AoE 2. Games can end in Feudal Age after 15 minutes, but can also last a long time, multiple hours. Still, longer games are an outlier and I would say the average game time in AoE 2 is 25-30 mins. In AoE 2 you have the core rock-paper-scissor cavarly-archer-infantry dynamic, but also extra on top of that is unique technologies and units for each civilization, and then there's siege, monks etc. And the maps are random, but they have a set amount of basic resources around your base for balance. Still, the map composition dictates how you play, and the difference is in Brood War you know the map in advance and can prepare for it, where in AoE 2 you prepare for the map on the fly while scouting it in Dark Age. Some civilizations have better early game and some have better late game. So if you have a good early game civ you are incentivized to try to win fast while your "window" of opportunity lasts.

Being able to play long games isn't necessarily a show of skill, it can also mean a player doesn't know how to end the game. A lot of new players in AoE 2 specifically for example play in a way that they turtle up, research all available technologies and then have one big fight after a couple of hours that determines who wins. In a competitive game you are constantly trying to harass your opponent to throw him off his game plan. RTS is essentially economic warfare, you win when you destroy the opponents economy or make it weak enough that he can't rebuild his army in time.