I'm really doubt if we will see any further 146 conversions. At the end of the day TNT is the only real player on this market but going to switch to 737s. 737s are more pespective freighters, just a bit over 146-300 capacity but offering more volume and much wider choise of airframes. 737-300/400 costs are falling down and keep in mind that 2 engines are always less expensive to maintain even if they are bigger, 146 is a really niche aircraft. Niche aircraft nowadays means expensive. As a person who is in charge with costs, analysis and all about that, today 733 costs the same money as 146 to operate.
If we are looking on commercial forecasts for pax-to-freighter conversions short-to-medium haul market, there is nothing about 146, it is all about 733/734/312/313/752.
If I'm a pilot I wouldn't put any single euro in getting 146 rating folks
P.S. Ofcourse it doesn't mean TNT will phase out 146s soon. There is simply no market to accomodate them, so they will be flying for some few years I recon.
edited for P.S.