There is clear economic incentives for all nodes involved with significant volumes of transactions to upgrade (less fees paid in agreggate). The adoption metric is not to be evaluated on a per-node basis.
As far as these large wallet providers are concerned the roll out could be immediate following the release of Seg Wit.
So the scenario above is far from "optimistic"
That non-consensual ('soft' fork) scaling 'solution' is a joke in the year of the halving. That's why the miners are finally acting.