Your concerns and pessimism are absolutely fair. I do have a pretty simple explanation that may ease your mind though.
Simply put - rewriting the runtime will take a lot of time and effort. Scirra feels like they have created something that is worth money to some people, and hopefully will finance this effort. After all they have been supporting C2 for years and years now. As a company, they also need income to operate and continue working.
So the alternative is wait and release C3 after a new runtime is complete which may come way later, or release what they have now and give users the OPTION of subscribing now or later.
They have been very responsive about engaging the community and clearly documenting their plans and thought process - to an absurd standard even. So I do have high faith that the new runtime will come eventually, as long as they don't shut down.
For any developer, it really comes down to opportunity cost and prioritizing what features to develop given the limited resource of development time - for example, do you want them to work on backgrounds for text that third party addons have already addressed, or do you want them to work on the underlying runtime which would open up a whole new realm of possibilities?