Ed, you make good points here.
l would also point out something. There are now enough examples of anarchosyndicalist unions as minority unions in some larger workplaces to draw some conclusions. l know of several times in the last 2-3 years where the mainstream unions had to be involved to get a strike going, but were not only willing, but actually signed deals without the consent of the workforce ... and then the anarchosyndicalists had open assemblies and workers from other unions agreed rather with the anarchosyndicalists and went on fighting, even after their unions signed deals .... and they got better deals. That's happened with comrades in Spain and in ltaly over the last couple of years.
Of course being in the minority can also be tough at times, but there is not any set scenario and certainly being a member of a bigger union doesn't guarantee you the best fight.
So while in one way what you say is true, and I don't think anarcho-syndicalists in 21st Britain would have a go at anyone joining a reformist union (in fact, I'd say most UK anarcho-syndicalists are also trade union members), I think your conception of what trade unionism looks like is not that representative of the reality.
So, firstly, I'd say that a lot of workplaces, in fact, in the UK and US prob the majority, have no union membership AT ALL and, depending on the age of the workforce, has a workforce with no experience of the labour movement or never even heard of 'trade unions' and what they're supposed to do. So in most of my workplaces, I've not only been the only 'radical' but the only union member/organiser as well.. at that point, even a reformist union is only gonna be able to give you much the same help as a radical group (i.e. outside support)..
Similarly, where I work now (public sector), there are quite a few union members but they still don't act as a union. Rather, they're in the union for individual representation. Again, the union can't really offer much more in the way of support in moving from individual representation to collective action. That still has to be done by those in the workplace, and the reality for most unions is that they're almost as absent from it (sometimes willfully) as anarcho-syndicalists..
So again, I think this division between being the anarcho-syndicalist loner at your work and being a union member with an immediate network of fellow members is, apart from in a few sectors, not the case. And where it is, there's nothing to stop people being members of both. A good account of what that would look like is Solfed's Workmates: direct action workplace organising on the London Underground..