Being totally reliant on a single carrier is not a good business plan.
Ryanair will not expand at PIK; The STN flights were the big money earner at PIK and until APD is scrapped (unlikely) Ryanair won't renter the internal UK market.
Until there is a decent business class lounge, no mainstream transatlantic airline will look at the place.
So where now for PIK? The management have regularly demonstrated they are incapable of doing anything with the place and being run by a bunch of civil servants on the new board of directors will not add to the commercial insight and imagination required to turn the airport around.
Would a Yes vote on September the 18th make a difference? Maybe. Maybe not. What you can be sure of is, absolutely nothing will happen until after the referendum.