I can't give you a hard number bud!  Those with the lowest capital cost of equipment and the lowest electrical rate in the simplest of terms. 

It is entirely plausible that in the future bitcoin mining will only be done by manufacture's as they will have access to the lowest possible priced units.  It's also plausible that at some point manufacturing costs begin to approach the unprofitable mark.  That is assuming tech flat lines as well as coin price.

Think about Bitmain and their cost to manufacture a unit.  That cost is A LOT lower than current retail pricing.  Combined with sub $.05 usd/kwh and you can see how easily they could outlast any retail purchasing miner for a LONG time in the event of declining coin price and a prolonged low coin price.  What happens if coin price falls to 4K and sits there for 5 years.  Most miners are betting against that as are most folks in this forum in general but it's not an impossibility!

It's also entirely plausible because BTC has the possibility IMO to go a lot higher in price that we see a much larger profit window for a long time to come! 

Until we can predict the impossible we can only speculate and ENJOY the fuckin ride!