The only reason I bought UE4 is because I can keep this Engine forever and all the sources and continue learning developing games at my own time.
marketplace will be accessible but during the time new content will be compatible only with higher versions
Upon the initial payment of $19 for the first month, you’ll get access to everything that’s been released by Epic thus far. That includes the compiled build/installer, full access to the source code and all sample packs / assets available on the marketplace. Once you’ve canceled your subscription, that doesn’t change. You still have access to all of that content - forever. The only thing that you won’t have access to is future updates that become available - which are pretty darn consistent!
The same goes for publishing. You’ll still be able to publish whatever you develop, no matter if you have an active subscription or not. Of course, the 5% royalties still apply. Though, that’s only assuming you’re project breaks the $3000 threshold
This is all explained in the ‘Unreal Engine FAQ’ which can be found here: Häufig gestellte Fragen - Unreal Engine
Hope that helps clear things up for you! 