Were liquid flyback boosters for the Shuttle ever realistic?

YES

http://www.ok1mjo.com/all/ostatni/sp...98377 048.pdf

Recovery costs would be dramatically reduced, along with propellant costs. LOX is $0.10 per kg and Kerosene is $0.40 per kg, whilst Polybutadiene and Ammonium Perchlorate costs well over $2 per kg. Recovery from the sea, versus landing at an airport, makes the SRBs way more costly than LRBs, the SRB has far lower performance than the LRB with the LRB being nearly twice as efficient, the cost of refuelling and handling the SRB is 10s times more costly than LRB, the ability to throttle the LRB makes things far safer for the LRB than the SRB, structure weight is far lower for the LRB than the SRB, increasing payload to orbit, haha - this is just the short list.