So pardon the plug, but finding a happy middle-ground to exactly this problem for Django based projects is what I now work on with https://www.codestasis.com/
Projects that can't upgrade, because of the ensuing cascade of breaking changes and dev time needed, subscribe to CodeStasis to minimally update Django to new non-breaking patch versions.
So you can keep your trusty old version yet also stay patched and secure if you find someone to do the heavy lifting for you at reasonable cost, which I think we deliver.