The Gemara is asking many questions where we see that the rabanim can override the torah
Why doesn?t the Gemara ask the following
We find in berachos 54a that in the times of boaz (inyanei deyomei) the were mesakein to greet each other with Hashem?s name
We see that the rabanim can be oker an issur of saying hashems name
Yehoash orange, Brooklyn ny usa
Excellent question from the other end of the Shas! I think the best answer to your question is that the Takanah to bless a friend using the Shem is part of a wider Sugya of all Berachos and prayers. We know that Chazal composed different Tefilos and Berachos, and it is obviously permitted to say Shem Hashem when praying.
The concept of the Takanah to greet a friend using the Shem is that this meeting has a higher level and can be regarded as a sort of prayer that Hashem should bless the friend you are greeting. Accordingly, this Takanah is not an Akirah of an Isur, but rather a wider definition and application of the concept of Tefilah.
Best Regards,
Aharon Steiner