Tosafos asks that according to Rashi that one can be yotzei by listening even during their own private shemoneh esrei, why would the gemara require one to wait until after kedusha before starting shemone esrei? Why not start shemoneh esrei and be yotzei kedusha through shomeiah k'oneh? Tosafos answers that according to Rashi although one can be yotzei through shomeiah k'oneh, it is not ideal. It is more of a hiddur mitzvah to actually say it and not be yotzei through listening.
It doesn't seem from Tosafos that we regard the listening as a type of hefsek lichatchila, rather the mitzvah of saying kedusha is not being performed ideally unless one actually says the words. Why is this different from kiddush on shabbos when we use shomeiah k'oneh lichatchila to be motzi others? If the concept of shomeia k'oneh doesn't work lichatchila, we shouldn't use it for kiddush either?
I was at a chasuna last night and posed this question to Rav Asher Weiss. He thought that although shomeiah k'oneh is only a "bidieved", there are other considerations that can override it and that is why we sometimes use it even lichtachila. I asked what is the consideration by kiddush that overrides shomeiah k'oneh, to which he said that since we have kiddush b'makom seduah, the group element is important. In my opinion this is not correct. Although the kiddush may require a makom s'eudah (be it d'oraysa or d'rabonon - see Rosh Pesachim), it does not require a group (or perhaps, as r. warren cinamon suggested to me it is just the regular din of b'rov am hadras melech that would override the lichatchila of saying it oneself). Perhaps by Megillah there is a concept of being yotzei b'tzibbur such as we find shelo b'zmano must be with a minyan, but by kiddush we don't find such a concept.
It seems to me that Tosafos is not saying a rule for all shomeiah k'oneh that it is only bidieved, to which kiddush is an exception. Rather, Tosafos is saying something very specific to the replying to kaddish and kedusha. Tosafos holds that since the goal of answering kaddish and kedusha is to actively participate and show that one agrees and confirms the greatness and holiness of Hashem, it is ideal to do this actively. Generally, the din shomeiah k'oneh is as ideal as doing it oneself, but when it comes to things that are meant to proclaim the holiness and greatness of Hashem, it is a hiddur mitzvah to do it actively by actually saying the words as a declaration.