I think what JfromB meant by 99% Perry music is that SP either created or co-created (with the exception of WITS & Faithfully) AND sang on every one of those songs from Infinity-TBF. Unless you are a very early- or latter-day Journey fan, that pretty much covers their entire catalog.
As to the point that, if anyone takes the credit, it should be the addition of JC who pushed the band to superstardom status. For sure, JC brought more sophistication to the songwriting. Suddenly, the songs seemed to evolve into more meaningful lyrical contents - that stuck to the core of the listener's conscience. But, SP was still in the writing mix (he just didn't have to shoulder the burden of coming up with all the lyrics). Plus, he had to vocally deliver them. And, I think John is right that, melodically, it was all SP. Which is probably why he got lead writing credit on all the songs he had a hand in writing and singing.
Another point, the J-boys toured relentlessly all through the '70s (HH saw to that), building name recognition with the masses. By 1980, they were already known as "one of the hardest working and most popular American bands." I believe they were on the cusp of superstardom anyway. Who's to say how Escape would have turned out without JC? We only know, for sure, there would be no Open Arms.
Lastly, and admittedly from a biased perspective, I believe SP can sing any of the songs on Revelation. I have only heard each of them a few times, so anyone ... refute if you have to. I don't believe any song is particularly vocal challenging to sing - nothing a 1997 SP (with the I Stand Alone song as an example) can't handle. Other than Lora and Cyndy (and his inner circle), none of us have heard the present-day SP sing. For all we know, he can chirp like a bird

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