The only way to make sure your blog will "last forever" is if you personally own the domain name it's on (e.g., example.com).
If your hosting service goes away but you own your domain name, you can bring up a new host that serves your content exactly as before by reproducing all the old URLs.
Otherwise, your content is dead. Copying it to a new host doesn't help. Every link anyone has ever made to you will go nowhere. The best you can hope for is links with good slugs so interested readers might search for your new home. Chances are they won't.
To actually commit to the claim of being "the most durable blog platform ever created" and "made to last forever", a blog hosting service should either:
A) Require new users to bring their existing domain.
B) Have users register for a new domain during sign-up.
Fundamentally, the service must support federation to be incorruptible infrastructure. Bonus if the service gives users a downloadable static version of their site so they can easily move with no broken links.