I have never been compelled to MAKE someone agree with me and the Sondheim discussion has been done to death so I won't try to convince you that his music will last long after he is gone. So...the following opinions are just MINE and I do not ask that you agree with me.
I am just curious as to how you can predict that he will be forgotten so quickly? I love Adam Guettel but he too can be uneven as are MOST composers. This is an art...not a science. Rather than compare or hold one composer over another, I would prefer to let them all grow and thrive and let history determine whose music will last.
As for whether Sondheim is creatively, technically or musically a great composer, many musicologists, critics and audiences have judged him to meet all the requirements. True, his compositions can be too complex, cerebral and dissonant for some...but that is taste. When judged on pure talent, it is hard to find him wanting IMHO.
And his "look at me" attitude? Sondheim is not a huge egomaniac who seeks attention and adoration. He does not compose for anyone other than himself. Yes, he is clever, and he uses the English language well. That does not mean he is being a "show off".
What I HAVE found is that many people come to Sondheim slowly...and that his music grows on them. Typically, I like to hear his music before I see the show if I can because I find I enjoy the music a lot more if I have had a chance to hear it first. There is so much to hear that you often fail to catch all the subtleties.
"Life is a lesson in humility"