WolfishMusings,
Your analogy is not valid.
We were commanded by God to eat matzah and marror on Pesach, while we were not commanded to do so on Shabbos.
Fine; I should have picked an analogy dealing with minhagim.
OTOH, building bonfires is not a command from God, but something that we, ourselves originated.
So should those who make bonfires do away with the practice? Should Ashkenazim stop eating latkes on Chanukah? Sefardim stop eating donuts? Should Chabad do away with Yud Tes Kislev? Fabrengens? Should we also do away with dates and pomegranates on Rosh Hashanah, wearing kittels on Seder night and Yom Kippur, making a siyum, drashos before mussaf on Shabbos, etc. because we cannot find them in Chummash?
As such, if we can do so for Lag B’Omer, I don’t see a reason why we can’t do so for Shavuous as well.
If you find a halachically acceptable way of safely making a bonfire on Shavuos, then feel free to do that, and eat cheesecake on Lag Baomer while you’re at it:)