All the money and resources should have been directed toward Colorado, Iowa, North Carolina and Alaska, states where the margins ended up being the narrowest. That's where the mistake was made, because if those states had been the main focus, we would have maintained them. Lots of resources were put into Arkansas, Louisiana and Kentucky, when it would have been better to spend that money elsewhere.
The map to a majority isn't in red states, it's in purple to blue states. Democrats did not go nuclear on people like Gardner and Ernst early enough, they need to totally tank their favorables early on, but they failed to do so.
Progressives spent over somewhere in the neighborhood of $50 million+ against Thom Tillis and attacked him hard early but it still wasn't enough.
What makes you think it would have been any different if the Dems had just concentrated on the states you mentioned? Esp. without hindsight AR, KY, and LA looked holdable.