I get what you're saying, really. It's just that more will, all things being equal, always be better. Of course. It's a no-brainer. It's more about being able to look at the package as a whole and go "is this worth my $60?" For me, the raw number of missions or planets don't really factor in. If they do for you, that's completely fine.
I played though Devils Lair like... 20 times over the course of beta, and would jump in and run it again at level 8 right now if I could. Running it once or twice might have been enough for you to get your fill, but for me, trying it out with different weapons, different skills, and different armors, as well as the variety of people I played it with kept it fresh each and every time.
Maybe to you, this sort of thing would get stale very quickly, but if the popularity of MMOs is anything to go by, a lot of people dig similar models, so it is not entirely fair to write it off. Not trying to pick you specifically, but there are a few people acting like someone must be stupid to be okay with the number of planets and missions. It's not that I want less for my money, it's that I feel that I'm getting my $60 worth. The quality, the fun, and the experience is worth $60 to me.
The strikes, along with 6(+?) more strikes, however many story missions, 4 more planets of varying size, PvP and raid, that is a type of variety that I feel comfortable paying for. I'm not too concerned about missions or planets because once I've seen it, I've seen it.
I'm not trying to get you to change your mind, but just wanted to say (though you very well might already know) that there are more ways to keep a game fresh and interesting beyond new things to look at. I'm just glad our exchanges have been quite civil. lol