Quoting Keynes in a lecture

Opinion, of course, but I think it is fine. It is often quoted in fact. While your students probably still think of themselves as immortal, they almost certainly aren't. No one should really take offense at basic biological certainties.

It is, in fact, a corrective on much illogical thinking, which is why it has lasted.

As a British person, I don't understand why you'd even consider that this quote might be inappropriate. If you feel that it's a good way to get your message across, there's no reason not to use it. However, if you're uncomfortable with it, for whatever reason, don't use it.

Writers often talk about the need to "kill your darlings". This refers to the fact that it's often easy to become enamored with one's own clever ideas and turns of phrase, whether or not they actually accomplish what is needed in the larger context.

Following this advice, I would suggest preparing the lecture without the phrase, simply to see how useful it really is. Come back to it a couple of days later, when you've had time enough to detach a bit, and see how well the lecture works. If the phrase is actually helpful and meaningful (as opposed to merely attention-grabbing), you can always add it back in then.