The company’s leading Wall Street critic, Bernstein Research’s Todd Juenger, raises the provocative idea this morning in a report that questions whether Viacom‘s stock — now hovering around its all-time high — can continue to rise. The share price has appreciated 55.4% so far in 2013 as investors grew confident that the company had reversed a startling decline in Nickelodeon’s ratings that began to show in late 2011. CEO Philippe Dauman says the turnaround largely reflects the success of new programs including Teenage Mutant Ninja Turtles. But Juenger says this morning that the ratings gains — and improvements in ad sales — are about to slow. Nickelodeon benefited in part because it began to target pre-Kindergarten viewers from 8:30 AM-2 PM — loading up on programs such as Dora The Explorer, Umizoomi, and Bubble Guppies — while Nick Jr focused on franchises “with arguably less audience appeal.” As a result “Nick Jr. lost about 250-300k average daily viewers [while] Nickelodeon gained about 100k.” Meanwhile, Nickelodeon squeezed SpongeBob SquarePants which now accounts for 45% of the channel’s programming hours, up from 25% in January 2012. “We know of no other network that relies so heavily on one single franchise,” Juenger says. It’s “undeniably risky to be so dependent on one franchise, especially one that’s 14 years old.” He adds that Nickelodeon also was helped by this year when Viacom began to stream its shows on Amazon Prime instead of Netflix. “This trade-off is a net benefit to Nickelodeon ratings…because there are more Netflix households than Amazon Prime households (who utilize the SVOD service).”

Analysts are divided on Viacom: About 15 have a “buy” on the stock while 16 have a “hold.” Last week one of the company’s bulls, Pivotal Research Group’s Brian Wieser, said that “optimism on Viacom has proven to be well-founded, as concerns about kids’ viewing trends have proven to be dramatically removed from reality.”