The bigger win I think for the `not` patterns will be anywhere you remove constructions in the form:

    if (!(e is Class)) {/*...*/}

Sometimes the ! gets visually lost in the extra parentheses. The `not` patterns should especially make it cleaner to write certain styles of DeMorgan substitutions (!a && !b to !(a || b) type things) in ways that will be easier to read.

Given those big wins, they alone provide a useful context of some consistency of having the `not` operator even for "simple" cases like (e is not null).