Haha, you don't realize that while yes, exponential growth won't last forever, the tech that's on its way within the coming years, particularly the 14nm (currently in engineering for mass production by Intel for its i-however-many-cores-this-time processor) and 10nm architectures (which Samsung demoed in 2012) will make everything from BFLs to ASICMINER Gen 3's mine like CPUs within a fortnight of their eventual release.

The only thing that's currently stopping this tech from happening in SHA256d ASICs is scalability - Intel and Samsung can't stay profitable in a niche market such as BTC mining by producing such cutting-edge tech.  The current mining companies don't have all the intellectual capital built up that these two tech giants do, but believe me, its only a matter of time before some senior engineer quits, their non-compete runs out, and they jump into the mining chip space.  Think what impact that'll have on the difficulty, assuming even that the number of people mining doesn't grow at all.