Hogan/Bischoff basically directly coincided with Impact’s downturn. They were legit an alternative to the WWE until they came in and made them WWE-lite.
The funny thing is TNA's best 12 month cycle in their history creatively (on a week to week basis) was under Hogan and Bischoff from BFG '11 to BFG '12 when Bruce Prichard had the book. Even Meltzer and them awarded Impact the best show of the year and he absolutely hated TNA at that point (probably because of Bischoff shytting on him at every turn). The problem is that LOLTNA had already settled in on the internet after Hogan and Bischoff's first 18 months.
TNA's real downturn had nothing to do with Hogan and Bischoff and everything to do with Dixie refusing to let Russo go (despite his booking tanking their PPV market) and Dixie's dad trying to saddle Viacom with Dixie when they wanted to buy the company. That was literally the only sticking point. Had he just sold TNA there and then they'd probably still be #2, a distant #2 like AEW or the other property Viacom owns (Bellator) but #2 regardless.