Your point is fair, however we do know there are stock piles of Geneseed hence Storm or Iron, which was written by Graham McNeil when it was his job to create the expanding narrative for 40K so it's a bit more solid than some BL stuff. Individual chapters also store a surplus of Gene seed the events in Blood Reaver cover this (and I think it's covered in a rules supliment as well). The imperium does have stasis technology that appears quite effective, Guilliman hasn't rotted in 8000 or so years, so storage may be possible.
Yes the Imperium is up sh** street but it remains to be seen what happens when the Golden Throne fails, will the demons from the web way destroy Terra, will the Emperor finally die and be reborn (he is a perpetual after all) or any of a myriad of things? I guess we'll only find out when GW really needs to drive sales 😉