Firstly old B737's that are not fuel efficient at all when compared to the B737-800.
This will deter many an astute business person.
What makes you think that?
Most business people couldn't give a rat's about how efficient their supplier is.
They only worry about the price the pay for the service or product they receive.
Give them on-time, high-quality, J-class service for a less than Y-class price, and some of them will give it a go. I don't think they'll be sitting there sipping their Seaview worrying about the J8TD9's gulping down the A1.
But if the age of the aircraft means reliability problems which lead to the schedule getting disrupted too often, then the innovators will soon go back to where they feel better served.
That's the marketplace at work...