"Spielzeug ist hingegen sinnvoll."
Should this not be "Spielzeuge sind hingegen sinnvoll" if the subject is "toys" and not "toy"?
Not necessarily. 'Spielzeug' can refer to a single toy and to a group of toys when they are viewed as a single group. If you want to stress the mess, 'Spielzeuge' does also work.
But why is "toy" wrong then? I guess you could make that sentence in English with "toy", or at least "the toy"