How do you define doorstepper? Something with long ROI, or something which cannot earn at least for its electricity?
VCUs electricity cost is still under 20% of earned amount for all the already released algos (and the more profitable algos are only going to come).
But difficulty is only ever going to go up as newer and better hardware is continuously coming along, which means that short ROI always turns into long ROI, and long ROI turns into a never ROI.
You forget ROI's can change dramatically with price of coin. Never know in that 2 year ROI BTC shoots back up to 20k.