You have it backwards. They are not successful because they are trilogies, they are trilogies (or more) because they started out as successful.
I like series for all the reasons you mentioned, but a game needs to stand on its own and I would actually be more hesitant to purchase one where I knew in advance that I will need to purchase a couple more to get closure - there are just too many variables and too much time between releases to be able to count on the next one(s) being as good as or even like the first.
In some cases, but not all. Either way, the end result is the same.
That was exactly the case with Mass Effect. Before the first game even came out in 2007, BioWare had already stated it was going to be a trilogy. That, in my opinion, shouldn't necessarily be a cause for hesitation. You just need to judge the game based off its own merits and see if you'd like to continue that story.
Epic sagas and personal/small-scale stories are quite the oxy-moron. You don't need 3 games to tell a personal tale.
I've never played through the trilogy without an imported protagonist either. But that's besides the point. As I said it was an intriguing feature, but inarguably trivial. You gravely overestimate the value of the import system, as it apparently blinds you from all of the other aspects of what make a good videogame that Mass Effect did well. I never suggested the trilogy be "3 separate games". I clearly stated that continuity and carry over is easily achieved without the import. I.e. the import is not essential.
Let's be honest though, they were 3 separate games with 3 different narrative styles despite the same protagonist. "Flavors" as you say. And what's wrong with different flavors? Variety is a good thing. I'm only one of many that are and have been ready to experience the MEU from a different point of view.
Your first sentence doesn't relate at all to the second sentence. Who says you can't have three games explain a personal tale? What do you think The Witcher series largely is? A personal story focusing on Geralt of Rivia. It has worked extremely well.
Playing Mass Effect without an imported character is a drastically different experience. I played Mass Effect 2 on PC as I got it for free after buying Dragon Age II on PC. I had Mass Effect on the Xbox 360, so I didn't have my imported character. It actually ruined the game. I couldn't customize and tailor every point and choice from ME1 and thus the Shepard I had didn't belong to me. It was a less engaging and less fulfilling experience as a result. That's why the save import matters.
You and I must have played a different Mass Effect then. The point of the trilogy was to always destroy the reapers. That had been the message since the very beginning and all the way to the very end of ME3. While the journey may have turned and shifted in different ways, the overarching goal was always identical. Mass Effect was always on target that reapers are bad and they must be stopped.
on the contrary seeing as DA is fantasy and Mass Effect is sci fi, I'd say it's quite an opportunity. They should distance themselves from being tied down by past installments. The implementation of the beginning/middle/end trilogy structure was shoddy at best the last time around. And I simply disagree that it leads to a more climactic arc. Standalone games allow for a much more branching narrative with much more opportunities for choices to have actual impact.
The Witcher series has already proved it and it'll do it again next month. All with the same protagonist. It is a trilogy of standalone tales. Quite unconventional. And it's swimming in success.
The Witcher series isn't three standalone games... TW2 had an import system for TW1 so you could import gear and choices... TW3 is also going to have a different kind of import feature at the very start where you input the choices you made in TW1 and TW2 to shape the story and to build your "world state." In other words, The Witcher trilogy is exactly like the Mass Effect trilogy. The only reason TW3 won't have a direct import is because of new consoles and it's not technologically feasible. Again, the Wild Hunt was at the very beginning of TW1, in much of TW2, and, surprise, it will be a major element for TW3. No difference.