The language we currently use in the amidah for בִּרְכַּת הַמִּינִים is different from that found in the earliest siddurim. The gemarah (Berachot 40b) teaches us that "If one alters the formula laid down by the Sages in benedictions, he has not performed his obligation". I realize that in the past using the original text of בִּרְכַּת הַמִּינִים may have posed a danger to life, however for many or perhaps all people today that is no longer the case. I would like to know why we do not revert to the original text of בִּרְכַּת הַמִּינִים.
Indeed, the text of the blessing changed. See here:
However, your question is based on assumptions that may be challenged, such that the following may form a response (or at least a challenge to bolster your assumptions more strongly than you have):
1) There is perhaps a difference between an individual deciding to change the text of the blessing himself, where it is not a blessing, and where the community, under the leadership of great rabbis of the generation, have changed the text of the blessing.
2) The gemara itself you mention has the context of blessings on foods. It needs proving that this extends to the blessings of Shmoneh Esrei.
3) Further, while the statement you cite (which is one side in a machlokes) has a letter ב, corresponding to עין משפט נר מצוה, such that it is brought down lehalacha, Rif seems to omit it, and Rambam (hilchot berachot 1:5) only says that he is erring, but does not say that it is not a blessing. He says about something else that it is not a blessing:
וְכָל הַמְּשַׁנֶּה מִמַּטְבֵּעַ שֶׁטָּבְעוּ חֲכָמִים בַּבְּרָכוֹת, אֵינוּ אֵלָא טוֹעֶה. וְכָל בְּרָכָה שְׁאֵין בָּהּ הַזְכָּרַת הַשֵּׁם וּמַלְכוּת, אֵינָהּ בְּרָכָה אֵלָא אִם כֵּן הָיְתָה סְמוּכָה לַחֲבִרְתָּהּ
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