How perfect is it that Magnus decides to take Alexander's surname? to unite it with his own, making it something unique?
Now, I'm not up to date with the books, but from what I gathered Alexander is it for Magnus. Before Alec, Magnus never married, never even bothered to imagine a forever with someone. Alec, in his simplicity and contradictions, has managed to tear down and entirely destroy that wall that had been up for centuries. And while Magnus desperately wants to hope for their forever, desperately wants it, he knows how it is going to end. He knows Alexander is going to pass someday and what he decides to do?
I can very well imagine Magnus never removing their wedding ring, even after Alec's passing, which would be a declaration on itself. But the surname?
He is not only saying "oh, he was my love, we were married once". He is incorporating him in his identity. We know how important identity is for a warlock, they usually choose their names for themselves. And Magnus is choosing to making Alec part of it, part of himself.
In a couple of centuries, when the present will only be a chapter in the book of history, Magnus still be Magnus Lightwood-Bane. He would still be Magnus Bane, but he would still be declaring, centuries from now, that he loved a shadowhunter once. That there's a part of him that still will be loving that shadowhunter. That he will probably never stop.
I don't ever want another love.
