Don't forget, Sesame Street isn't just a show, it's a laboratory. As long as the research is current (and PBS doesn't run out of money) they should be able to continue for a long time. Besides, if the show were in danger of going away, we wouldn't be seeing all these ancillary materials (Beginnings, etc.)
Besides, if you think what we've got is not focused on the older characters, talk to some of the people who grew up with some of the international versions. The US still has remaining from the first episode:
Five frequently appearing muppet characters (two still played mostly by the original performer, two occasionally by the original performer, and one whose deceased original performer still shows up in occasional older segments)
Three human characters (a regular played by a different performer, a semi-frequent appearer played by the original performer, and an occasional character played by the original performer)
And two buildings.
Sure, it's not the same show we grew up with, but it is still, in my opinion, a strong show. It's just that it's a laboratory, not a museum.