Estonia and Finland can't secure baltics that well, we don't have that much equipment. US air forces is needed also in southern if war occurs between NATO and Russia, northern countries likely covers air mainly by themselves. It's not about can we fet, but will we get supplies trough Sweden. Uncertainty is not good thing, with NATO membership, it is certain.

And Sweden can sign their own treaties, but will they?

But you pretty much need Sweden to get those arms into Finland, and Swedish navy to secure baltic sea. And together Sweden, Finland and Norway has very strong airforce.