Rabbotai,
The gemara tried to make a comparison from the tenufa and shaving of the Leviim to Nazir. Why would such a comparison be valid? The inauguration of the Leviim in the mishkan was a one time affair never to be repeated. It wasn't l'doros. Therefore, it doesn't seem that it should be compared to something that is l'doros.
B'kavod,
Sam Kosofsky
Being that the Gemara concludes with a Tzad Ha'Shaveh anyway, there is no longer a question based on the Levi'im not being l'Doros. The Gemara does not always ask every possible question, especially when the question would be dealt with by the conclusion of the discussion. However, it definitely would have been an appropriate question if the Gemara would have answered differently, as is apparent from many places throughout Shas.
All the best,
Yaakov Montrose