The thing is, GDI currently only knows that there is more underground Tiberium than we expected.
OOC, we know that part of the path of growth of Tiberium, as expected by the Scrin, is that eventually it will build up and explode in something similar to what Kane arranged to happen at Temple Prime. And Ithillid has said that if we work at it with harvesting, mitigation measures, vein mines and boreholes, we can keep the Earth habitable for a while, possibly up to a century or so, but without a TCN it's not really possible to save it.