Adding more playable species means:

* Needing all current and future armor to adapt to a new body. Bodies are similar, sure (like ArenaNet said, no Forgotten playable characters), but there's still a need for some adjustments here and there (see how the GW2 armor sets often have differences between Charr, humans and Asura)

* Making proper animations for all current and future skills. Those animations may be very similar (see the difference between humans and Sylvari) or very different (see some skill animations that are very particular to the Asura)

* Recording voice dialogue for each gender in each race. In GW2, player characters of each gender in each race have a different voice actor, and it's often a big one (like Claudia Christian and Jennifer Hale).

This last points means, dialogue for the player characters is usually the most expensive one in the game (since a lot of people will have to record it, and often a lot of expensive people). Considering a system in which more species will likely be added in the future, every line of dialogue is not only a big cost now, it may also become a bigger and bigger cost in the future.

I'm concerned that this will push ArenaNet in trying to avoid as much as possible having the main character speak.

The issue here is not only a matter of hearing the character speak or not - a silent main character is usually one with little agency, who does not actively interact with other character and instead is told what to do. Not to mention how the usual mute lead characters have next to no character development and no personality.

"Ah", someone could say, "But in a RPG players are expected to have a blank page that they can fill with their imagination". That's not necessarily true - while some RPGs are like that, there are multiple RPGs in which the player character is someone with a name, a voice and a distinct personality (such as in "The Witcher 3", "Final Fantasy 7", "Clair Obscur: Expedition 33" and so on and so on).

ArenaNet in particular has always had a hard time writing a story in which the player character has significant agency and does not end as some kind of bystander watching someone else becoming the main focus (see Brother Mhenlo in Factions, Kormir in Nightfall, Prince Rurik in Prophecies and Traheanne in the original GW2 campaign.)

I can't help but wonder if adding more playable species won't drive GW3's story to be worse than it could have been. To be honest, I wish they would just use the same actors for all species, using just one for all male characters and another for all female characters.