IS grew in the mid and late 1960s as a very amorphous revolutionary tendency. Its main paper in the early 1960s, Young Guard, was more anarchistic than anything else. But the group which had dominated the revolutionary left for the previous period, Gerry Healy’s SLL, obliged the IS by becoming increasingly mad and suicidally sectarian, for example boycotting the big demonstrations against the Vietnam war in the late 60s.
Despite the caricature Marxist idea that material reality decides everything, there is also a cultural reality, which has an autonomy. The previous political cultures in a working class, social-democratic, Stalinist, or other, have an autonomy.
The culture does not change easily. In fact the culture on the revolutionary left didn’t change much, because the IS group adopted much of the culture of the previous SLL, as sea-creatures crawl into shells. As the SLL declined in the 1970s, its culture was taken over by the IS group.
That autonomy of culture is in fact what Trotsky summed up in the idea of "the crisis of leadership", or it’s a facet of the same thing. Workers tend to stick with the organisation that educated them or first brought them into political life, and new activists tend to take their political culture from the main body of established activists which they first find. That is true within the revolutionary left, too.
Not many people survived politically from the wreckage of the SLL, and possibly not many will survive from the crisis of the SWP.
But IS grew from the mid 1960s through the early 1970s. It had resources. It raised quite a lot of money because it had quite a lot of well-off members.
In 1971 SWP (then called IS) had 115 branches, with an average of maybe 20 active members per branch. Today the SWP has 93 branches, and a branch is considered thriving if its attendance is in double figures. The SWP’s growth, or lack of it, since the early 1970s gives little basis for boasting.
Despite its exaggerated claims, the SWP has maybe a thousand active members, and another thousand or so people who pay dues and occasionally attend SWP mobilisations.
The SWP seems more prominent on the left today because the groups which in the early 1970s were bigger than it (the Communist Party and Healy’s Workers’ Revolutionary Party), and those which then were smaller but visibly in the same league (the IMG and Militant), have imploded or collapsed. The Militant (continued as the SP) has revived a little since its low point of the 1990s, but those other groups exist today only as small splinters.
The SWP today, also, has built up a large income (it has quite a few old-timers who no longer do much activity but have well-paid jobs), and so can finance several dozen full-time organisers, whose activity makes up much of the profile of the SWP.
The SWP has had ups as well as downs since the early 1970s. Its status as the most visible group on the left has enabled it to recruit, over the years, many talented activists. However, that it has avoided implosion since the early 1970s is no vindication of its political twists and turns.
And it holds good only until larger political tumult either throws the SWP into crisis, or creates a radicalisation lively enough to give raw material for other groups to outstrip the lead which political inertia gives to the SWP, or both.