I disagree. I think there are a lot of first-party characters that could be interesting fighters.
This is true. There are several awesome 1st party gals to choose from, and I and many others have listed them many times in the topic. However... Aside from male/female avatar pairings and duos, Samus and Bayonetta are the only women who are standalone main characters in the entire roster. More importantly, part of the reason that there are still so many awesome 1st party gals to choose from is because a lot of them have been overlooked. Smash has historically put women on the backburner and old Nintendo practices are partially to blame. Basically, 3rd parties help level the playing field.
Furthermore, when you look beyond Nintendo, there's just way more iconic leading ladies (especially when it comes to indie games, for example). Although we'd have our Elma, (K)achi, Officer Howard, Ribbon Girl (and other ARMS gals), Anna (and various other Fire Emblem gals), Medusa, Toadette, Barabara the Bat, Jill Dozer, Dixie Kong, Style Savvy rep, Ayumi Tachibana, Yuri Kozukata, etc. We'd lose our Carmen Sandiego, Chun-li/Sakura, 2B, Estelle Bright, Kat, Amaterasu, Katalina, Jade (Beyond Good and Evil), Shepard, Morrigan, Tracer, Reimu, Sarah Kerrigan, Miriam, Ciri, Faith Conners, Arle Nadja, Ellie, Bella Goth, KOS-MOS, Joanna Dark, Elizabeth, Heart Aino, Cooking Mama, Yumi, Shantae, Chell, Sakura Shinguji, Aloy, Gris, Lara Croft, Jill Valentine, Sophitia/Cassandra Alexandra, etc. Leading ladies from critically acclaimed games, many of which from games considered some of the best games in their genre who are in various "Greatest of all time" lists.
The inclusion of 3rd parties opened the doors for a lot of awesome leading ladies and it'd be a heck of a lot better than what we've been getting in previous Smash titles. Thus far, the only woman we've gotten so far is a bird that's used a weapon. I think that is what she means by missed opportunity.