If you've been following the latest Drupal developments, this may come as a surprise! Drupal 8 has, relatively speaking, only just began to make an impact. Furthermore, Drupal 7 is still very much alive!
Druapl 8's launch has been a rocky one, mainly because 7 was so very good, there's been a lot of catching up to do in terms of contributed modules. Whilst nearly there, it's still not quite there for more complex projects and would take something of a leap of faith, or a lot of bespoke programming if you have very complex requirements. In the majority of scenarios, it's a very sound choice.
Drupal have decided that a shift to 9 (launching mid 2020) will facilitate a move away from 8's dependency on Symofony 3 which is reaching end of life (end of 2021). The good news is that we are promised a smooth upgrade from 8 to 9.
We believe there are still choices to be made when setting up a Drupal website. Both 7 & 8 are good until at least November 2021. Our advice is, where possible, use Drupal 8 for an easy transition to 9. This should make for greatest website longevity.