people are defending anna because anna is the only character in the series who can truly represent the entirety of it. no matter where you started (unless you somehow had the misfortune of only playing gaiden), anna is a recognizable face. that she's one of the three main characters of fire emblem heroes, the title where a massive part of the appeal lays in crossover fanservice, or that she's chosen as the face of FE's social media, is not a random coincidence. she may not have the upfront popular appeal that marth or ike or lyn enjoy, but she's a familiar and consistent element of the franchise that fans are generally happy to see included, and as closely associated with the franchise as any of them. in a game that's been heavily criticized for not being fully representative of FE as a whole you wanna get rid of the one character that does just that?
and you don't need to be 'happy' about it but you don't have to be incredulous about why people like seeing her or how she deserves to be in any more over whichever random army character you liked from a previous title. don't get me wrong either, i love tons of random army characters. the large majority of the characters i like the most are random army characters who will never actually get acknowledged outside of a game like heroes. but you can either admit that you want a roster that caters to your wants specifically and not pitch it as who actually 'deserves' to be there, because anna has around 13 games worth of reasons (edit: 14, i forgot #FE was a thing for a second. maybe i'm forgetting more, lmao).
and honestly, a character who not only has an entire history of a franchise behind her to draw from, but also already has a unique concept in a game where truly unique concepts are hard to come by, as a gold-hungry merchant, she has numerous game reasons why she'd be more introducing to include than the large majority of random units you might prefer over her.