TODAY’S COLLEGE STUDENTS GREW UP in a world where cable TV and AIDS always existed, the sun rose in the east and NBC dominated Thursday nights with something called “Must-See TV.”And with that, say goodbye to the age of network birthrights. These days, everything is earned, and even successful programs hang onto that status more perilously, with a greater sense of fragility, than in the past. Listen to fans: Despite their loyalty to various series, many derive an almost perverse glee from proclaiming that a show has “jumped the shark” in the same way they savor seeing the fabulously paid sports icons they cheer receive a comeuppance. Watching NBC’s presentation this week spurred reflection on both the miscalculations that went into the diminution of its Thursday-night lineup — now coming up on the short end of the ratings wars to CBS, with more formidable challenges boding ill for next season — as well as the network’s denial, or spin, in publicly acknowledging the enormity of that loss. NBC’s list of development sins goes back so long there’s really no one to blame, which probably won’t stop at least the knee-jerk impulse to throw someone overboard at the first sight of a shark fin. Like any primitive tribe, Hollywood has its rituals. Yet any accounting of the present “Must-See” malaise must take into account the mediocrity of past occupants of that precious real estate — including “Veronica’s Closet,” “The Single Guy” and “Union Square” — along with the occasional preciousness of “Scrubs” and the conceptual narrowness (sad but true) of “Will & Grace.” THOSE STUMBLES, coupled with the modern allure of a short-term fix, inspired current management to slip “The Apprentice” into that night, a solution with resounding long-term consequences. Not only has that unscripted program diminished this season, but with Thursday’s comedy block halved, NBC finds itself with no engine for generating new sitcoms or gaining traction for them. NBC officials also dismissed the notion of a square-one renovation of “Joey,” which fails to address that the show was broadly sampled and soundly rejected, meaning drastic procedures are in order. Personally, I’d have Joey open season two on the phone with his sister, saying how sorry he was that the L.A. move didn’t work out. Either that, or there’s always the old “bad dream” trick. NBC Entertainment prexy Kevin Reilly correctly assessed the challenge sitcoms face, noting Monday that unlike dramas, “Comedies rarely self-start without some nurturing and time-period protection.” Where sitcom hatchlings can find either, alas, is the question nightmares are made of. Clearly, NBC isn’t alone in this regard, but its situation provides the starkest reminder of the huge uphill climb required to make even “Maybe-See TV” a reality again. The sun is shining over Manhattan this week, and yes, it rose in the east. After that, kids, it looks like a dark and stormy night for comedy. COMEDY CORNERED, PART II: As a critic, it’s difficult not to harbor mixed feelings about the renewals of Fox’s Emmy-winning “Arrested Development,” ABC’s “Jake in Progress” and NBC’s “The Office,” which garnered considerable praise but by virtually any measure failed to deliver in the ratings. Given the sorry state in which comedy finds itself on the major nets, there’s perhaps a greater rationale for extending well-made programs with cable-sized audiences, hoping the positive response will somehow translate into higher tune-in, as opposed to simply rolling the dice all over again. Nevertheless, part of the process feels particularly futile, taking a flier on prestige series in a peculiar kind of welfare toward the fashionably talented rich. It remains to be seen, moreover, what concessions might be extracted in the push to render these shows more “commercial,” as sadly happened to NBC’s ill-fated drama “Boomtown.” Nice as it would be to see this trio succeed, history indicates the Grim Reaper’s scythe can be delayed but seldom diverted. Even HBO has discovered that not all its children can be rendered hits despite its proclamations to that effect, as the moribund second-season episodes of its showbiz romp “Entourage,” returning next month, would suggest. Until the fall, then, critics and quality advocates can bask in a rare victory. Any celebration, however, should be tempered by the knowledge that unlike pay cable, the networks are TV, and must inevitably adhere to their own unyielding set of rules.