I definitely disagree with the statement that SupCom is a "traditional" RTS. I have seen many discussions about games taking too long. In fact, I believe Chris Taylor originally envisioned a large game of SupCom taking a weekend to play

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In fact, the way Sins is evolving is following SupCom's evolution fairly closely. With the release of SupCom's stand alone expansion, Forged Alliance, the designers have taken much of the communities feed back to heart and made the average game take a bit less time.
The neat thing about SupCom is that games can take 10 minutes or they can take 6 hours.
Also, you will very often have more than "one" base in SupCom. Even on medium sized maps players often make forward "firebases". I definitely agree that Sins is a unique game but making comparisons to other aspects of existing RTSs does have value.
Games taking so long in Sins' is definitely a problem but I think it is essentially separate from the slippery slope problem. A game that takes 2 hours from the very start to the very finish is likely decided much much sooner where the last 45 minutes are simply filler. The main problem here, I think, it that it takes a very long time to bomb down a planet. A player can decisively win a battle, which make the odds of that player winning very high, yet it will still take 45 minutes, even if the loser doesn't even try to rebuild his fleet

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I think the trick is, is to make the actual defining moment very late in the game. If a game only takes 45 minutes an observer watching the replay should not be able to tell who is going to win at the 23 minute mark. Just because a player wins the initial first battle, it should not mean they win the war. It should give them an advantage yes but not so much so that the other player(s) can't make a come back.
I think Shadow said it well:
"some actions should have a profound impact in the game, but while changing it, an opponent should still have more means to cope with it other than just taking back a certain planet at any cost to avoid long term defeat."