Do you mean in terms of resolution or other performance aspects ?
In terms of resolution they might almost always be the weakest link, there is probably quite a signal loss when it is converted. The better the source and electronics the better speakers will sound even if they relatively become more and more of the weakest link.
So, generally speaking, I would reserve the term "weakest link" for everything but the speakers. Speakers are very much acoustic instruments and should be treated differently.
In terms of resolution they might almost always be the weakest link, there is probably quite a signal loss when it is converted. The better the source and electronics the better speakers will sound even if they relatively become more and more of the weakest link.
So, generally speaking, I would reserve the term "weakest link" for everything but the speakers. Speakers are very much acoustic instruments and should be treated differently.