How do you save generated content not tokens in emails & print/merge?
The feature seems to be available just it seems very few components use it. For example if you do a contribution search and then choose to send thank you letters from the actions dropdown, it DOES convert the tokens in the filed activity on the contact.

In the code if the caller of this function includes the $perContactHtml parameter, then the activities get filed with the tokens converted:
https://github.com/civicrm/civicrm-core/blob/5.18.4/CRM/Contact/Form/Task/PDFLetterCommon.php#L278

The contribution one does do that, e.g.
https://github.com/civicrm/civicrm-core/blob/5.18.4/CRM/Contribute/Form/Task/PDFLetterCommon.php#L127

and here is where it resolves the tokens:
https://github.com/civicrm/civicrm-core/blob/5.18.4/CRM/Contribute/Form/Task/PDFLetterCommon.php#L431

But it seems most of them don't and so it uses the default which is just the verbatim text.