Looking at these charts it seems to me the historical rate of digging has been close to staking (the latter being mostly a constant except for the various rule changes). There have been periods of slow digging, and periods of faster digging. Perhaps that is somewhat a function of price action (slow digging -> less supply -> price increases -> more digging -> lower prices -> slow digging, repeat), or perhaps just independent fluctuations (or both).

Eventually staking has to dominate because the potential digging is finite, but that may not happen for a long time.

I believe effective decentralization will drive the price higher as the coin becomes more trustworthy for everyone. That can be part of a virtuous cycle where more decentralization leads to more trust and more adoption which leads to more decentralization. I also believe this can potentially be undermined by "voting" systems that are inherently centralizing (share of voting power is an increasing function of share of stake). That's somewhat similar to dooglas's point about delegated stake being potentially dangerous. I guess we'll see how it works out.