How far below is the question. It could level out at 60% - that is believable. However it can't level out at 99% - Somewhere around 95% major sites will decide IPv4 isn't worth supporting and they will just ignore that final 5% of customers, which will force them to upgrade - which in turn will give others confidence to remove their final 4% of customers - until IPv4 dies.
There are also still Telex and X.25 networks around there, not to forget the whole public telephone network!
But at some point, getting a native connection to all of these started becoming increasingly rare, and now these are largely emulated/tunneled on top of IP. The same can happen for IPv4.