Are there any PaaS providers out there delivering Craft as a platform continuously updated and secured?
I'm not aware of any PaaS service that also keeps Craft up-to-date for you automatically.
There are some like Fortrabbit, that are PaaS and are very familiar with Craft installs, but you'd still be on the hook for updating Craft.
There are some more traditional, managed, non-PaaS hosts like ArcusTech that are also very familiar with Craft installs, and they keep the OS configured and secured for you are responsible for installing and updating Craft as well.
There is some more discussions about hosting options for Craft here: Recommended hosting company for a Craft site?