Evidently, upgrading from Drupal 7 to Drupal 9 is a major undertaking. What happens, though, if you don't?
First, you may relax knowing that your website will keep working. However, there won't be any further module or Drupal core releases or bug patches after reaching Drupal 7 end of life 2023. Through the Drupal 7 Extended Vendor Support program, a few restricted security patches will be available through November 2025.
However, as time goes on, you will lag behind new developments and find it harder to satisfy the expectations of your consumers.
Secondly, as the community-based Drupal Security Team ceases keeping an eye on Drupal version 7 end of life, security will become more of a problem.
Finally, any plugins or other integrations from third parties may stop working when their developers switch to technologies that are incompatible with Drupal 7.
Maintaining Drupal 7 may seem the simplest option, but it will use up more resources and jeopardize the goals of your company.