Oh man I have a lot of thoughts about this. I totally agree with everything you've said, however I can also understand people's reticence to dwell on grief, it's a difficult and overwhelming emotion. I know there are a lot of fans who want to believe that postwar Levi was able to live his best life, unencumbered by grief, and enjoying his freedom with Gabi and Falco. And I also know that there are some fans who are really offended by the idea that Levi would spend the rest of his life grieving, never moving on from those he lost during the war. Certainly living a hollow, grief stricken life is a terrifying prospect, however grief isn’t that simple. One of the best explanations of grief I’ve come across said that when you lose someone you love, you don’t just forget them, or replace them, time can never heal that wound or fill that void. However with time and patience your life can grow around that absence and expand to encompass new people, new love, new experiences. The absence is always there, it becomes part of you, and your life continues to grow around it. I think Mikasa is actually a good example of that. It’s clear that she continued to love Eren for the rest of her life, but that didn’t stop her from loving other people and going on to raise a family. I don’t think she ever “moved on” from Eren, no one could ever replace him, but I do think she was still able to find love in the world. The Ackermans were fierce and constant in their love and their loyalty; both Mikasa and Kenny remembered their liege, their chosen person, to the end of their days, and I see no reason for Levi to be any different. I believe Erwin’s absence would have left an indelible mark on Levi, and I don’t think anyone could ever replace him, however I also believe that Levi was able to forge a new found family with Gabi, Falco and Onyankopon and I hope he was able to end his days peacefully.
If you're interested, I've written several fics about Levi's life after the war that explore his grief for Erwin and his fallen comrades, and his new life with Gabi and Falco.