When all that remains is heavy/sour, who's going to build/refurbish the refineries and where?

It's being done. Lots of refiners are installing cokers, primarily to take advantage of a large light/heavy price differential. Hydrotreaters/hydrocrackers are also being installed to handle sour crudes. However, it is a substantial capital investment, and with margins where they are some of these projects are likely to be delayed, and some that are still on the drawing board may be scrapped. If there was certainty that last summer's margins would return, everyone who doesn't have a coker would install one.