When this question is asked, there's always a hundred people eagerly telling you to switch to Fastmail, forgetting that 14 years ago, everyone was saying switch to Gmail because it was the good option.
I would just suggest people think about what might happen if Fastmail gets enough of a userbase to be the next Gmail.
14 years ago, Google was a very different company. I don't think it's reasonable to criticise people for not seeing what they would become that far in advance.
I agree with you, I just caution that they may not have become what they are today if we didn't all flock to Google. Fastmail is good today, will they be good if they become the dominant player?
I would just suggest people think about what might happen if Fastmail gets enough of a userbase to be the next Gmail.