I think the next bottleneck is likely to be software longevity. This has improved a lot lately!
But basically I think phones have now reached a place like laptops and desktops where HW capabilities are stable, and there's no good reason for devices to be obsolete in 5 years any more.
I wonder if regulators said "your software has to get security updates for 10 years" would that be enough to get mobile SoC vendors upstreaming their HW support and Google streamlining the Android update process? (To Google's credit they have already done a _lot_ of positive work in that area as I understand it).