This is... one of the dumbest things I've read here.
And not just because I disagree with you, OP.
You want to make the game more BC like - flying mounts were a major novelty of BC. In fact, that they were end game was pretty much a major factor in people grinding out to get flying in order to begin it. It was THE thing to start working on when you hit 70, even more than starting heroics and doing raids.
Secondly:
So you want to remove heroic... so there's just normal. And then you want to replace normal with flex? Or do you want flex and normal to be the new normal and heroic? Either way, you're dumbing down the raids, and making it more "casual friendly" which is against what you initially want.
But let's address the LFR issue - you want the game to go back to BC. Back to a time when the majority of the players capped at raiding Magtheridon, Gruul, and Karazahn because they weren't the top 2% of players who regularly raided. Therefore, you want to DENY the casuals the chance to raid. Do you understand that the casuals make up the majority of the player base? You are essentially ostracizing the majority of subscribers... and you're expecting millions to resubscribe?
Moreover, many of the subscription declines are due to either gold farmers being booted, or perhaps (crazy idea), the older gen who got into WoW at the beginning have simply moved on because it's been almost a decade.
Finally, the old subscribers didn't leave because of LFR. This is completely illogical. If LFR replaced regular raiding, I can understand your thinking, but it didn't. The hard core game from BC is still available, there's just an extra mode for non hard-core people.
Removal of flying mounts and LFR will do nothing to inflate the subscription numbers. I can't even imagine what would make you think otherwise.