You complain about ME3's ending because there was no "Yay we bet the reapers, now let us ride off into the sunset" (which I think would have been the worst possible ending. I rarely choose destroy because dying at the end of ME3 fits my Shepards), but then criticize DAI because that was the ending it had (at least without trespasser). There are some people who like to criticize or "hate on" things just because it's trendy or edgy, and they look down on anyone who doesn't share their views/opinions. They think it makes them seem more artistic/worldly/special doesn't, it makes them seem obnoxious and small minded.
He's not complaining the DAI had a "Beat the bad guy, go home and party" ending. He's complaining that this is the only ending. Just as ME3 only has a bittersweet," The Shepard dies for our sins" ending. (Yes, yes, breath scene nonsense).
Having a real variety of endings would account for both. And given both games have a "power" system which measure's your organization's strengths, there's no real reason there shouldn't have been such a variety of endings.
I'd go a somewhat diferent way with that. I think the issue is that Bio doesn't actually think they are in the wish-fulfillment business in the first place, even though that's how their choices typically operate. The only thing the DE plot and the final plot have in common is that they were both going to offer only choices with some bad consequences to the player. This makes sense if you think that the games were about making hard choices, but as we both know, the games are about avoiding making the hard choices -- unless you're into having a bad outcome this time through.
It's not about wish fulfillment. It's about telling a coherent story. The story itself might be a wish-fulfillment one. But that is not necessarily the case. It's about having the freedom to tell a happy or tragic story depending on the choices you make. As it is, no matter how perfect a run Shepard has through the three games, he or she is going to hit a brick wall with the Catalyst. FailShep is the one the ending caters to. Not a successful one.
Similarly, no matter how well or poorly the Inquisition is built up, the Herald of Andraste is going to kick Corypheus's @$$ and go home. You can't even select Cole to go with you on the final run if you haven't done his personal quest (as he would still be vulnerable to being bound by Cory otherwise)
What are you actually saying should have been done with Synthesis? They should have somehow lampshaded the moral unacceptability of the choices? Or is that impossible, and they should have made the consequences worse so players wouldn't get the idea that they're doing the right thing?
Synthesis should never have existed as an option, let alone as the "ideal" one. Moral acceptability aside, it was never something the trilogy ever explored, save perhaps in Reaper reproduction.