Just to be clear, the cost of changing the iterations is almost zero. Bitwarden already supports variable PBKDF2 rounds on all platforms, as well as changing the number of rounds. I'm sorry, but the cost of deploying Argon2 in production to a lot of platforms across a lot of devices is non-trivial by comparison. If you have enough different combinations of devices and environments deploying an if statement can become a challenge. In this case, it's especially a problem considering the lack of Webcrypto support.
By all means, use Argon2 in new code, or any other more modern KDF. But PBKDF2 isn't broken, and replacing it warrants actually doing the ground work to see if it makes sense: is it fast enough on most devices? is the security improvement meaningful enough? etc.
The truth is, 1password has the right idea here with their Master Key system. Even very unwieldy long passwords are pretty low entropy compared to a proper cryptographic key, and KDFs cannot significantly improve this situation. If you want to do more than re-inforce the speed bump, you're going to need to work outside the password. It has a usability trade-off of course, so maybe it's good that not everyone does it that way.