The 40 second timeout isn't just FANS, it is for both FANS and ATN and is a feature of the ATC system as we cannot have hundreds of messages just sitting there awaiting action that may never come. Your reason for timing out is exactly the issue, the message can just sit there behind other messages and not get responded to. Because of the way FANS is, there is no way of deleting the message from the ground so you will always be able to read it even after our timeout, but with ATN any unactioned message is removed instantly. The spiel can be something like "BIgJet123, disregard CPDLC message for RFL, disconnect CPDLC and continue with voice comms" and then we have to go back and ask/instruct whatever the timeout message was. Then you wait for the inevitable "Say again London???" hence the reluctance to use it with FANS.