It's not a mistake.... Brahms of ALL composers knew exactly what he was doing
I would tend to agree. I've heard some far out rationalizations for changes like this. For example, the low FFFF descending line toward the end of the 1st mvmt of Tchaik 6, where the tenors drop out and the bass continues. A retired principal of a top 5 orchestra taught one of my colleagues that the tenors should play this, "because the reason for it was that tenor players in Russia at the time weren't good enough to sound good playing that passage." I'm paraphrasing from a secondhand account of course, and something may have been lost in translation. I was rather flabbergasted that my friend was asking me to play this.