Again, though, that's just a really senseless argument.
You're repeating the "if you can't avoid every single American product, then anything less is pointless" line but, now, immediately contracting yourself by saying "anyone with half a brain is already shopping local as much as possible". Why is "as much as possible" sufficient in one context but not another?
You are also pretending, despite all evidence to the contrary, that the boycott efforts consist of a handful of Mumsnetters. That's plainly untrue.
For example, here in Canada, the Premiers of our Provinces (including the right-wing ones) have also been taking actions in support of the boycotts, as well as generally encouraging them and, as I've already posted, 85% of Canadians report that they either are/will be buying Canadian more often or (wherever possible) avoiding US products in particular.
Obviously, the follow-through and the extent of efforts will vary, but that's some 35 million people in Canada alone who are engaged.
As an aside, one nugget from that survey I posted is that 48% of Canadians have said that they will cancel or delay plans to travel to the US (and it's noted that Canadian travel agencies are seeing people follow through). As I'd previously posted, thr US Travel Association estimated that even a 10% drop in Canadian tourism would cost the US more than $2bn per year, and result in around 14,000 job losses. It looks very much like the actual drop will be far larger.