Is this mixture of plural and singular legitimate?

But what is most important for our purposes is that these changes were the signal for the resumption of historical debate on a grand scale, of the kind that had been suspended or driven underground during the reign of Louis XIV. （The French Idea of Revolution ed. by Dale Van Kley: 203)

I am interested in the expression "these changes were the signal". Shouldn't every change be a signal, so that there are many signals? Why use the singular form? Because all these changes together form a signal, and one or two just cannot signify that there is a resumption of that debate?

A verb agrees with its subject, not its complement, not even in copulas.

His injuries that day were the start of his decline.

These ideas are a fresh new way of looking at the problem.

My parents are the only reason I bother to go home for Christmas anymore.