$15 minimum wage = what unionized market would pay at very bottom

Were the US labor market fully unionized, the lowest negotiated hourly wage would probably clock in at around $15. *   Setting same as the minimum wage in that fair and balanced market would not make a lot of difference.
   
In the case of a no minimum wage and no unions at all market, labor would be subject to subsistence-plus pricing -- each bit of extra employee ability is rewarded with a bit more pay – w/o reference to how much consumers might actually fork over.
   
In today’s actual US labor market the federal minimum pays about half what my theoretical unionized market above pays at bottom -- $7.25 – with next-to-no unionized private firms, compared to other rich economies – 6.2% (and dropping).  Simply raising the federal minimum wage to $15 would mostly subject labor to $15-plus pricing – would mostly not provide labor across-the-board with the mechanisms to collectively extract the max the consumer market would be willing to pay.

Today’s US labor market 40 percentile wage is about $15.  Raising the minimum wage to $15 would only assure 40 percent of earners what a thoroughly unionized market might pay at very bottom -- not accomplishing much if we are serious about building (rebuilding) a fair and balanced US labor market.
http://fortune.com/2015/04/13/who-makes-15-per-hour/  

Should Republicans win back the house while holding on to the senate and white house they must certainly foist a ratchet down labor law – like the one they had in the hopper last congress, requiring union recert/decert elections at every private (non-gov) workplace where union membership has rolled over 50% since last certification. 
https://www.congress.gov/bill/115th-congress/house-bill/2723/text

Assuming the Democrats take back the senate next year, why (oh why?) shouldn’t they enact a ratchet up/ratchet down labor law – requiring periodic union cert/recert/decert elections at every private workplace as SEIU lawyer Andrew Strom has put forth?
https://onlabor.org/why-not-hold-union-representation-elections-on-a-regular-schedule/

* A fairly robust guesstimate: 1968 federal minimum was $12 – at half today’s per capita income -- meaning consumer support was there.
https://data.bls.gov/cgi-bin/cpicalc.pl?cost1=1.60&year1=196801&year2=202001