It's one thing to believe that flag burning should be illegal, but is it really worth the time and effort of amending the Constitution over a minor issue like this? It's not like flag burning happens that often anyway.
If the Supreme Court had upheld anti-flag burning laws, I wouldn't try to change the Constitution to invalidate the ruling (though if an amendment was to be introduced, I'd support it).
Flag burning may offend people, but so does hate speech. Why should one be legal and the other not?
Flag burning seems like a minor issue to you, slavery is the same for me.