That's because flush toilets responded effectively to a real human need: how to dispose of human waste in urban areas. Once that problem is solved, further innovation is mainly wasted.

I expected a better pun...

What's problems are we facing that are as important as removing human waste from urban areas?

Let's see... Traffic fatalities, to become extinct within 30 years or so, as self-driving vehicles take over. Also will transform the transportation and urban landscape to unrecognizability.

Poverty is another one; but it's not like we know of a specific technological innovation that would solve poverty

You can't "solve" poverty, the distribution will always be there, it's statistical mechanics and probability at work. Not even the former soviet union, with no unemployment and with the standard of living near uniform, was able to bring up everyone to this admittedly low standard.

Maybe solving death and disease

"Solving" is a strong word. But cancer research keeps chugging along, with new ideas popping up at least as fast as before. Augmentation, miniaturization and automation is progressing well, too. The latter would be much further along if not for the many regulatory issues. Medicine is ripe for an industrial revolution, with surgical conveyor belts replacing master craftsmen and their apprentices.

Even if we had teleporters, would future Tyler Cowens be writing that they're not as innovative as the car - and would they be correct, in that a teleporter is just a more efficient way of solving a problem that cars and airplanes had already partially solved?

That's just silly, if someone claimed that. There is more difference between an airplane and a teleporter than between an airplane and a rickshaw. Imagine, no roads, no travel time, no humans clustering into cities and countries... Unfortunately, there is no indication that teleporting is anywhere on the horizon, we don't have any clue about the new physics that would allow that.

As for something as revolutionary as the birth control, responsible eugenics is getting closer to reality, and is held back mostly by the prevailing hypocritical morality.

I suspect that this OP is sort of a devil's advocate and you personally don't believe Tyler Cowen that the pace of innovation is really slowing.