It is fairly easy to estimate how much transactions can be handle by a global usage. It's not that much speculating but yes, it is still a limit but at that point alternate solutions (AKA lightning and sidechains + shared market cap of other major altcoins) can come into play. 8 Gb is fairly enough to ensure a great amount of transaction fees to keep a large incentive to miners and ensure a maximum network security.
Security of the network and the idea of there being a network in the first place, depends on how many full nodes there are, how diversely they are spread across the planet and how able they are to handle the load.
5 super-connected super-nodes with 8Gb blocks would technically still constitute a network, but I'm not sure if you want to call that Bitcoin.
We need to address the block size issue responsibly and re-evaluate network's health with regular intervals.
I too still sympathize with the idea of one-off fork to 8Mb. Clean and simple.