The 124 definitely has it's niche, but for general cargo it's operating costs are far higher than for the 747F.
I do recall hearing of a program to build a new batch of updated 124s with CF6-80C2 engines and updated electronics/avionics. But I don't think anything ever became of it.
Less Hair, I agree that 747F does have the segment of the market to itself, and many of the 747-400F/BCF's out there are getting seriously long in the tooth (as in well over 100,000 hours). Those can't keep flying indefinitely and will eventually need to be replaced. Furthermore, there are not many passenger 747s left out there that are not similarly 'well used' to be converted to freighters.
But at the same time, Boeing is still a business - not a charity. At current production rates the 747F is at best a break-even prospect, and to keep building them beyond the current order book is going to require a major investment in new fuselage manufacturing capability. Such an investment doesn't make sense unless Boeing can be reasonably confident of enough future orders to not simply keep the current production rate but to increase it by at least a factor of two.