TLDR: Drupal 7 has a much longer lifespan than the (already pushed back) official date, and Drupal 8 has an essentially infinite lifespan because it updates in-place to Drupal 9 easily (and the same will be true of Drupal 10, 11, ∞.). There's no reason to rush an upgrade— but there's no reason to wait either.
That's the short version.
A client recently wrote to Agaric about Drupal 7 / Drupal 8 / Drupal 9 project planning questions:
With the EOL for Drupal 7 in Nov of 2022, and the EOL for Drupal 8 in Nov 2021, is there a reason we should move a D7 site to D8 at all this year? Seems like we might want to move directly to D9? We don’t want to feel pushed up against a wall with a “new” site build in Drupal 8, if we can limp along in D7 for a couple more years while we develop a D9 site with a longer lifespan. I’m wondering if you might have time to discuss pros and cons briefly so we can get a good plan together for moving forward.
I started typing and almost did not stop:
No one believes me when i say this, but i repeat my assurance that Drupal 7 will be well-supported commercially until 2030 or later (Drupal 6, released in 2008, still has semi-official long term support until at least February 24th, 2022— and Drupal 7 has a larger install base than Drupal 6 ever did, and currently has the largest install base of any version of Drupal by far, with more than half a million tracked installs.
Drupal 7 will be supported by the community for a long time. You do not have to feel pushed to a new version, like, ever.