Everyone and everything eventually expires. In the 1940s there were movie starts that people today have never heard of. People pass away and eventually others in future generations forget about their legacy. This is also true for certain types of music or for other fads that come and go. Everyone and everything is terminal.
Greg Lake's passing does not spell a death blow for progressive rick - that genre of music died long ago, long before deaths of Bowie and others who pioneered it in the 70s. Progressive rock was a niche genre that appealed a only limited audience even at the height of its popularity, and there were few who pioneered it successfully and brought it to mainstream or managed to expand beyond its boundaries. ELP had a brief period of popularity with one Top 40 single, but their talent and virtuosity went largely unappreciated from the mid-70s on. Ask anyone under 50 if they've ever heard of Greg Lake or ELP, or bands like Kraftwerk or Edgar Winter Group, and you're likely to get an overwhelmingly sad response. It is somewhat ironic that the man behind one of the most tolerable Christmas songs died during the holiday season. RIP Greg Lake, there are a few who will miss you. Progressive rick - it died long ago.
As with any genre of music, there will be holes in the hearts of many due to the loss of such an important part of this music. I do believe that this genre will be able to continue to succeed without him. As long as there are people who enjoy this music , it will continue to exist.
While the passing of Greg Lake is sad, he is not the only prog rock genius to have existed. Pink Floyd (although perhaps not with the true lineup) still exist and release music to this day. There are newer bands such as Porcupine Tree which still continue to have a strong audience. Perhaps Prog Rock is no longer in the main stream, but it's still got blood passing through it's veins!