That would have been just as much of an asspull as the crucible, there is literally no way the galaxy could have practically won unless the Reapers had like 10 in total numbers.
Ok, if you wanna get real for a second, it's less about being absolutely plausible and more about being satisfying for the player. If it was about being realistic, the Crucible wouldn't've existed either. But since it's all a wild fantasy anyway, you might as well understand where that desire comes from and play to certain strengths. Shepard IS already a Deus Ex Machina for all life in the galaxy, and the ultimate mac daddy/mommy. He/she is beyond conventionality, digging into the problems where no one else can find the answer, and coming out with the victory. Shepard wins Shepard's way. Shepard is the Anti-Reaper. Shepard does the Impossible. Row row, and all that jazz.
You still have the conventional battle, and that affects who survives and which races have triumphant moments (or don't) --- outside that, you have a lot of ways to end it by personally confronting the Reapers. You can talk your way into victory, you can shoot your way into victory, you can sneak and sabotage your way into victory, you can sacrifice yourself to victory, you can sacrifice
others, you can win with logic, you can win with heart, you can win with apathy(!). You can go it alone, or include your team. And these can be mixed and matched together. Preference by method of personal principle and gameplay style should dictate. As long as the player keeps the story, and their past actions, in mind.
But I suppose I'm holding the ideal Mass Effect ending to Chris Avellone standards, when that's actually... utterly impossible considering Mass Effect's development ecosystem.