god, just imagine treating this thing like a real actual individual game and not just as a point of comparison against the original
Pay $60 for an RPG where your characters do not have resolved arcs until later games that release years from now 
I don't have to imagine. I played and bought Xenosaga and Mass Effect games.
I haven't played Xenosaga, but in Mass Effect 1 basically every party member (sorry Kaiden) has a very clear beginning, middle, and end to their character arcs, the problem being that they kept recycling those plots in the sequels (garrus is ethics cop, tali getting over geth baggage) or pulled status quo changes out of their ass because their arcs were done (writing out wrex, ashley, and Liara)
Mass Effect doesn't leave a whole lot hanging. all the named antagonists are dead, shepards crew have had their demons dealt with (with the implication that the aliens are going to go their seperate ways afterward), and the galaxy is largely at peace.
What I'm saying is, if Mass Effect ended 1/3 of the way through its story, with Saren and Sovereign still alive and all the party members midway through their deals (and ending, I dunno, after that dumb planet with the big mind controlling spore monster) rather than being a complete experience on its own, mass effect would fucking suck.