You don't think a story that centers around the struggle against a cycle of mass genocide that has continued for untold millenia doesn't lend itself to a bleak ending? I can't agree with that. As far as I'm concerned the problem with the ending has nothing to do with its bleak tone, and everything to do with the encounter with the Catalyst and how poorly that fit into the larger scheme of Mass Effect's story. Aside from that of course, you also have the problem of the underwhelming manner in which your prior choices are represented in the climax.
Since day one, the series has been about Shepard overcoming impossible odds. Since
before day one - the Colonist and Earthborn backgrounds are about Shepard overcoming some pretty long odds even as a kid, and the War Hero and Sole Survivor military histories are explicitly about performing an action so outrageous and outlikely that it places her at the absolute top of the list
of the entire human race to be considered for a prestigious, symbolically important position.
It only escalates from there. The second game ends with Shepard literally surviving a Suicide Mission*, potentially without sacrificing a single squadmate or crew member. The first act of Mass Effect 3 involves Shepard curing a plague that has wounded an entire race for
more than 1400 years, and healing the wounds that drove apart two of the primary species involved. There is a happy ending for everyone involved, so long as you choose that path. The second act involves making peace between two species who have been mortal enemies
since the literal moment of the genesis of the younger species, to the point where they form a symbiotic relationship and almost immediately begin advancing forward together faster than they ever could working alone. It is an unambiguously happy ending.
Not only has Shepard been doing the impossible since about fifteen years before the first game even began, but the extent to which her actions have resulted in a happy ending has been
increasing, from 'Just barely holding on to her own life/Escaping a hard life on the streets' to 'Managing to hold Elysium with heavy losses/Staying alive while the rest of her unit is slaughtered' up to 'The Geth and the Quarians are doing together in months what it would have taken them decades to do working apart'.
The specifics of how dangerous the Reapers are isn't really important. From a storytelling perspective, they are simply the
most impossible of all the impossible situations Shepard has faced down. They're the climax of the story, and the last, greatest trial that Shepard has to overcome. To be thematically consistent with
the entire franchise up until that point, it must be a decisive victory. "No matter what you do, you throw the entire galaxy into a dark age that is guaranteed to kill
many times over the number of people who were killed by the Reapers up until that point" is not appropriate as a mandatory, "this always happens no matter what" component of the ending. Having the Mass Relays blow up should have been in the game as an
option, but it should have been the Mass Effect 3 equivalent of dying at the end of the Suicide Mission, or at
most coming out of the suicide mission with the bare minimum number of surviving squadmates. The "best" ending should have involved galactic society picking up the pieces and moving on, with no further shaking up of the status quo. That wouldn't even be a super-happy, "Disney" ending - it still doesn't bring back the untold
billions of people who are killed by the Reapers between the start and the end of the game, the colonies and planets that were razed, the families destroyed, the cultures lost, etc. Ignoring all of the messages of the franchise and contradicting nearly all of what should have been foreshadowing for the end, in favour of an ending that essentially kills the galaxy even if you do absolutely everything right is just completely unwarranted, and embarrassingly poor storytelling.
Put another way: This is the 'sad' ending equivalent of having Shepard find the seven magic Dragon Balls and ask the dragon Shenron to bring back to life all the people that were ever killed by the Reapers, and for his second and third wish to also bring back Mordin only he won't be old anymore and to fix Thane's lungs and download Legion's personality into Shepard's space coffee mug.
*(And I'll point out here that it doesn't really matter if you don't think the suicide mission seemed all that dangerous, or if you don't think the Collectors are as threatening as the Reapers, or whatever. The first real trailer released for the game started with something like "I'm collecting specialists for a suicide mission...", it was used everywhere in the game's marketing, it's on the back of the box, that's how characters in-game are constantly referencing it, and it's what it's actually
called by the game. In story terms, symbolically, it means "You are not meant to survive this", and yet Shepard does - and not only that, but can ensure that a
dozen playable characters, as well as every named and unnamed member of the ME2 Normandy crew, survives along with her.)
