Indeed! Where there is money there is a solution usually. It will take some time, but if orders for FPGA rise at prices the sellers make good money, production increase will follow.
That doesn't follow at all, hence the GPU shortages. Increasing manufacturing capacity costs money. Having that capacity unused also costs money. GPU manufacturers decided to not increase capacity because the risk of mining demand dropping is too great. FPGAs are no different, except starting from a lower base, a drop in demand and dumping on eBay would result in even greater losses later on for the manufacturers.