The default is very obviously “no”.

Fine by me, I am not arguing that the default must be "yes", but that you need a default.

If someone does not agree to the new terms, they did not agree to the new terms.

True. Problem is that from a company point of view, it is better to handle the (supposed) few exceptions that the (supposed) overwhelming normality, that why this questions are posed this way: you simply pose the question in a way that you minimize your work.

I can’t send you a new set of terms for reading my comments and just assume you forfeit your firstborn child to me if you don’t answer within 10 minutes.

A little excessive as example, but on the other hand I can argue that if you not put a time limit on these type of changes or decisions, I can simply say today that your 10 years old wedding is invalid because I have some reason to challenge your 10 years old marriage publications. (To explain, in Italy if you want to marry you need to put an announce, usually for a couple of weeks, on a public space, normally the town hall building, to let people who has some valid reasons to challenge it)

Also, normally in Italy a change in the agreed terms has only two options: you accept the new terms (even by silent consent) or the agreement is no more valid and it is dissolved without any penalties if present, so what 23andMe is trying to do is formally correct. But a big dick move.

I'm guessing there's some kind of terminology in the original agreement that says something like we reserve the right to change this agreement at any time and you agree to be notified of any changes by the email address on your file.

Disclaimer: I am not a lawyer