One could argue that the decline started back around TAR11/12. But it was very slow and incremental. Perhaps, negligible even. Interestingly, I feel like the transition from SD to HD is when the show's real steady decline started.
From TAR18 to TAR23, you could see the casting and the Leg design was definitely not at the same level as any of the SD seasons before. And the ratings reflected that. While the rest of broadcast primetime was hemorrhaging viewers, TAR had been one of the few consistent shows. They had maintained their ratings, NFL overrun boost or not, even when almost everyone else was declining. But it was around this time when TAR's ratings really fell and not even NFL overruns were enough to pad the numbers.
TAR24 was just the straw that broke the proverbial camel's back for many reasons. And of course that decline led to TAR moving to Fridays. Fridays did not kill TAR. TAR was moved to Fridays in the first place because it no longer was doing as well as it once was on Sundays.
The Friday timeslot certainly helped speed up the ratings and popularity decline. But the "quality" decline was steady and increasingly so from TAR26 to the present. Budget was obviously lower, but shouldn't be too much of a hindrance in terms of Leg design. TARs around the world have far lower budgets, yet can still produce seasons better and more enjoyable than TARUS did during this time. So TARUS had no excuse.
Casting, also one of the most important aspects of the show, continued to be subpar. And the gimmick seasons did not result in good, memorable teams. Nor did they bring in any new viewers either.