It's not Bischoff/Hogan. They were on a good run of like 3-4 months before Bischoff/Hogan came in, but before that they had a couple years of hit-and-miss TV and PPVs with some really stupid booking at times.
They had some real momentum throughout 2006 that they fukked up, and I think this is one of the times where you can actually blame Russo. They were building momentum all throughout 2006 to the point that there were TNA chants at WWE shows during bad segments, they had back to back PPVs do 50-60k buys, they had 3k people for BFG, then a few things happened. They did a bunch of heel turns with Joe/Angle that didn't really work, then they made Christian the heel champ and AJ Styles super goofy. The tag division that was so hot during 2006 became just OK when Styles/Daniels and AMW broke up, and LAX cooled off some due to working with nobodies or teams just collecting a check (Dudleys).
And then Sting vs Abyss. That was their top angle or co-top angle from the end of 2006 to spring 2007 and it sums up their downfall at the time. Fire Russo chants during a match with a crowd that usually loved everything, Abyss' mom coming out, so much goofy sh1t. 2007-2010 when Hogan came in they stayed around a 1, but their momentum was gone, they didn't go any further, probably because word of mouth stopped being good. If a million people watch and most people love the show, the audience will grow. If a million people watch and most people think the show is OK, hit-and-miss, or watch out of habit, then the audience isn't going to grow.
The Hogan years had some good points here and there, they also had sh1t like aces n 8s and Joker Sting though.