The entire point in using a massively successful commercial platform for this kind of thing is that spare parts will continue to be in production for decades to support the huge customer base. Even if the -8 has a limited run compared to other 747s, it's not like they've dropped support for the SP even though it too was only built in limited numbers.
As for why the 747 could be discontinued, at least for awhile; there's an upper limit on the number of superjumbos needed on the planet, and I expect that we're probably not far from that point. So long as the current fleet continues to operate safely then there's simply no need to produce more planes. As the current fleet wears though, eventually new replacements will be needed for those routes where moving this many passengers makes sense, especially if the manufacturers can get the efficiency up. That's part of what's eating into the superjumbos; the ability for multiple flights a day with smaller planes to get equal fuel economy per passenger and at the same time offer more flexibility (ie more than one flight per day) due to the use of smaller planes.
My wife used to fly fifteen times a year. There was one city that she flew to the most, and she chose the airline with the most daily flights because airlines will often move one up to an earlier flight or two that same day if there's empty space, because they can sell the seats on the later flights to last-minute purchasers. She could come home four or eight hours early if she was done early and didn't need to be there anymore. An airline flying two or three 777s or A320s per day offers her more flexibility than one flying one 747 or one A380, and that's worth something.