Isn't this already true? You are allowed to flaunt copyright under certain conditions, including if the portion copied is sufficiently small, or if the new work is considered transformative. I would imagine that many instances of sampling would be protected because of this.

SoundCloud's responsibility should be to defend the rights of their users. If a DMCA claim is filed in a way that SoundCloud deems to be unfair, then they might offer to litigate on behalf of their user. As it stands, the majority of artists will capitulate to studio even in cases where the law is on their side, because they haven't the means to defend themselves.

Yes and no. The limit is something like 4 bars I believe. That may be a sensible limit for the traditional 3:20 minute song on the radio, where such a snippet can often capture the central motif of the song.

But most of what makes SoundCloud different are the 3-hour DJ sets that can draw on hundreds of different sources, including other songs, or movie dialog, or noises recorded in nature.

There's no absolute truth how to value each and every contribution to something like that, and it may well be fair to keep some royalties flowing to many of these sources. But as far as I understand it, the current regime is somewhat lopsided in the original creators' favour.