Synthetic diamonds are not easy to produce and they take time and energy (literally). The only reason synthetics are made is because the price of real diamonds help prop up the price of artificial ones. If the market collapses, as you predict, than artificial diamonds don’t become economical to create any more. My prediction: new mines will continue to be found and real diamonds will have the same market hold they have now in 20 years.
Research on bulk synthesis of diamond will continue for the foreseeable future, it's just too important a manufacturing material. It will be a massive win for thermal engineering when semiconductor chips can easily be embedded in a diamond matrix.
