Why the core migrate modules won't be enough? I've seen that Drupal 8 comes with 2 core migration modules; These are "Migrate", and "Migrate Drupal".

My question: if we want to migrate SQL from a Drupal 7 site to our brand new Drupal 8 site, why aren't these two enough, and why do we also need to download the module Drupal Upgrade? Wasn't there a plan to make the process free of any contrib modules whatsoever?
This is asking two questions. 1) What do I need beside core for migrating to D8 from D7. 2) What is Drupal Upgrade.


If all your fields and data in D7 are core fields, then you do not need anything from contrib. If you have non-core fields i.e. userpoints, then you'll need source and destination migrate plugins for those non-core field types. Same applies to any non-core Drupal data.
Core does not currently have a UI. It is in the works, but until then if you want a UI, use Drupal Upgrade. Or you can use drush and migrate_tools. Mike Ryans's blog post provides a  break-down of the current module landscape for D8 migrations.


Lastly, if you are trying to suck in strait SQL data, the above mentioned blog post provides some suggestions.


  In migrate_plus, open up migrate_example/README.txt and it will guide
  you through the basics of implementing SQL-based migrations. It's
  simpler than you think!