Yes, that's a big statement in the title, so let me explain.
Lots of OSTraining customers are looking into Drupal 8 and they have questions about Drupal 8's future. If they invest in the platform today, how long will that investment last?
This is just my personal opinion, but I think an investment in Drupal 8 will last a long, long time.
Drupal 8 took five years. It was a mammoth undertaking, and no-one in the Drupal community has the energy for a similar re-write. Drupal 9, if it ever does arrive, will be a far smaller update.
We're seeing Drupal developers get comfortable with much smaller releases. Drupal 8.1 arrived in April and had new features including, migration improvements and the BigPipe module.
Usability improvements for blocks may arrive with Drupal 8.3.
There's already talk of a new core theme with Drupal 8.4.
So, Drupal is adjusting successfully to the new reality. Big releases are probably history. Drupal's future is in regular, small, backwards-compatible releases.
And Drupal 9? We're more likely to see Drupal 8.9 before we see Drupal 9. And that's a good thing for Drupal.
Update: Gábor Hojtsy, who lead Drupal 8's multilingual initiative, wrote a response. Gabor's ideas sound similar to the ideas in this article, "the first Drupal 9 release could be the same as the last Drupal 8 release with the cruft removed."
Update: I've been wrong many times, but did get this one right. Six months after this post, Dries has said:
Drupal 9.0 should be almost identical to the last Drupal 8 release ... Updating from Drupal 8's latest version to Drupal 9.0.0 should be as easy as updating between minor versions of Drupal 8.