Technically you are incorrect. Many lords are non-obligatory units. Lyn, Hector, Ephraim, to name a few and several non lords are forced units. Avatar and Lucina being non obligatory in the story does not make them any less valuable as lords. Chrom only appears to take the obligatory role in story mode but he actually shares it with the Avatar and the game is very upfront about it. The Avatar also takes the lead in spotpass whereas he does not, the avatar is both important to the story and meant to represent the player. Much of the game focused to expand the Avatar hence they are much more involved than their beta versions of FE12. Chrom is also not as involved as the avatar to the overall goal of the game. Chrom hardly hogs that "only lord" title to himself. The only reason he's in the game is because Lucina reversed his death and in return causing a retelling of the story. Yet the bigger goal of the game was that of Avatar resisting or giving in to the assimilation of evil that their father set them on.
A second discussion i'd like to talk about is that several of the characters already in smash bros are as "second fiddle" or even less important than people claim non-Chrom lords are. Much like these characters, Lucina achieved the high popularity fan status and as a bonus she is the developers' favorite lord (between Chrom, Avatar, and her). People scraping her chances because she "looks" like a female Marth and yet ignore Chrom looking like Ike are unjustly turning a blind eye. I am aware that Lucina can easily be a costume of Marth but the same argument holds true for Chrom simply remove Ike's bandana and give him a recolor to match Chrom's palette. I strongly believe Chrom is not a necessary lock to represent Awakening. I know i'm making a large gap and comparison here, but think of Twilight Princess. Midna was the major character in that game but she was not required to transition that to smash bros. It could easily be the same case here, Marth indirectly or not is an icon of Awakening. In reality much of it was advertised because of "Marth's" return. I think that Chrom's "support" reeks of people that expect and not of people that actually like or even know him. Is his SSB fanbase an actual fanbase? And is it really justifiable to use as material to discredit the Avatar and Lucina? I believe not.