Not quite. ISPs have not been investing in IPv6 because they had no IPv6 traffic, current or projected. Should all of their customers been on essentially IPv6 through regular upgrades, their investment in IPv6 would come before IPv4 address exhaustion, because all of their customers (and pretty much any other host in the world) would already be ready and _fully_ _configured_ for IPv6, IPv6 addresses are cheap and content is _widely_ _available_. In other words, everything is ready. Why would they then need to think about several layers of NAT (how much is that going to cost?) and other nonsense? IPv6 would be fully ready for them by switching their gear or even just upgrading software.
The reason this did not happen with the current plan is because IPv6 is the "other" thing that causes endless headaches nobody wants to think about with no benefit in sight. There is no content right now, nobody is configured for it, so why risk supporting it?.
Imagine an ISP in this v4/v6 embedded scenario that did not follow what I'm describing above and persisted on IPv4 only support on their equipment. With all of the end points ready (and this is where the value of the net is), they would be simply swallowed by the ones that invested and were able to attract new customers en masse, because they could offer better connectivity (visibility of new and old, point to point comms, no NAT etc.).
Yes, it's dollars in the end. ISPs like to have customers. They would have had a lot less trouble keeping/getting those should the alternative plan been followed.