There is no financial incentive for most companies to maintain extremely old URLs. There is no large visible cost known to the project manager at a company who chooses to migrate to a different CMS.
A common solution is to create placeholders at the old URLs with permanent redirects to the new URLs, but that doesn't fix all of the old hyperlinks, so it becomes extremely expensive to maintain backwards compatibility after 2 or more CMS changes.
Comments
There is no financial incentive for most companies to maintain extremely old URLs. There is no large visible cost known to the project manager at a company who chooses to migrate to a different CMS.
A common solution is to create placeholders at the old URLs with permanent redirects to the new URLs, but that doesn't fix all of the old hyperlinks, so it becomes extremely expensive to maintain backwards compatibility after 2 or more CMS changes.