Their platform is whatever they call “Wordpress”. In the same way, NYT on a phone app is conceptually the same NYT as the one printed in 1923, even if almost nothing in the production pipeline is the same.
Wordpress is the technology, if it changes, it becomes something else.
That's the difference, explained.
They own the Wordpress trademark just like NYT owns theirs. Somebody who buys a 100-year registration from Wordpress will have the service delivered on whatever Wordpress has become by 2123.
People use WP because it's a technology they understand and can put to a particular use. If that changes, said people may not use it anymore.
If the NYT decides to change their content delivery platform to, literally, whatever, people would still go read the NYT. Their audience doesn't care.
If WP becomes a different product (a static site generator for Rust, deployed with Kubernetes?), their customer base will drop to ~0 overnight.
There are lots of examples of software platforms transitioning to a new foundation while retaining a compatibility shim for those bits of the old applications that customers still need. Text-mode COBOL applications are deployed on Windows 11, which itself has almost nothing in common with Windows 1.0.
Wordpress.com would need to maintain compatibility with some PHP+MySQL plugins. This doesn’t seem insurmountable, especially as AI code translation is now a thing.
You're imagining this change happening in a world where people still care about Wordpress.
Try imagining instead, a world where Wordpress is already a dead technology that nobody would use for a greenfield project, and it's only "legacy" sites that haven't been modified in 10 years (but which are owned by brick-and-mortar companies that continue faithfully paying the hosting bills every month) that are still hosted on Wordpress.com.
Can you not then see the value in some company buying up Wordpress.com, and migrating its customers over to a Wordpress-compatibility-shim over their own service that's based on a not-dead technology?
Just because Wordpress 6.3 is a php monstrosity with a security-disaster-riddled ecosystem doesn't mean Wordpress 12 has to use php and can't have sandboxed extensions. For all we know, Wordpress 12 could be a fancy frontend for a static site generator (marketed as "ultra-caching" or whatever)