The point is that the definition of "practical" is debatable -- any reasonable person can see that there are more risks associated with mucking around with the circuit board than having Apple install a custom software build, which carries no technical risk at all.

It doesn't matter that you can come up with some theoretically plausible attack that works in this one case. If it's harder or riskier or slower or less effective than Apple complying with the warrant, then the question stands.