Just to throw in my two cents as a member of the old SDS.
In my opinion, the decision of the currents of the far left to abstain from the SDS was one of the worse of the 1960s. Contrary to how it generally seems to be getting discussed here, the SDS was not a disciplined cadre organization. Pretending that this or that current that was in the SDS WAS the SDS is simply a misunderstanding of the reality. In practice, much of it paid very little attention to "the leadership" in the national office.
The political level was abysmal, and the dominant belief that somehow the Democratic party would managed to muddle through into some new radical manifestation was groundless and silly. And some disciplined socialists could have done wonders about that. Then, too, over the past thirty years, I found the political level of some socialist cadre organizations to be not that much higher, and the hardly free of some of the same silliness. And I'm not talking about the DSA, which obviously inherited much of this.
Why a more coherent socialist current hasn't emerged as a more active force within the DSA also eludes me.
In the end, the choice between participating in an overtly socialist current within a mass organization of radical young people or throwing everything into spending thirty years turning a group of 6-8 people into a group of 6-8 slightly different people seems to me to have been a no-brainer. Even on the most pragmatic level, I think that the organization would do better all around as a current of some influence in a significantly larger organization.
I am unfamiliar with the new SDS beyond seeing it as not the old SDS, despite similarities.