The Moment SVU Went Off the Rails Happened All the Way Back in Season 5
Well that's if you believe that SVU ever went off the rails – the show still remains popular and it even became the longest-running primetime live-action series in the history of American television.
True, over its lifetime it witnessed two notable and enduring dips in ratings, the first after Season 7, and the second after Season 20, but a lot of people are still clearly watching it.
However, there was a clear point, fairly early in the series run, when SVU discarded any pretense of believability and authenticity, and it might be connected with the first ratings drop.
Yes, pretty much nothing about the policework, or the legal procedures, depicted on SVU was ever realistically portrayed at any point.
Except perhaps cops beating up suspects and otherwise breaking the rules and suffering no real consequences for that – but even then, probably not with the frequency at which such incidents occurred in SVU.
But at first the show had remained relatively believable, and consistent in its internal logic.
However, at some point – what point exactly the fans of the show still argue, but possibly as early as during Season 5 – that just went out of the window. "Insane storylines with a high taste for histrionics" appeared and became increasingly common.
The last pretenses of even caring about believability and consistency were definitely dropped no later than the time Mariska Hargitay got involved as a producer (which happened during Season 16).
After that, the show completely embraced all sorts of stupid tropes, related to police work, from launching investigations based on "vibes", to police officers always having all the time in the world to work on a case-of-the-week no matter how shaky it is, to criminals getting caught and sentenced without fail (in early seasons some of them got away with their crimes).
As to whether all of this truly damaged the show, that's hard to say, particularly as no specific moment of radical departure from its earlier traditions can be pinpointed, only gradual shifts in tone.
After all, people don't exactly expect meticulous realism from police procedurals, they expect some combination of good guys catching the bad guys and nerve-wrenching drama.