If the question is "Does lower fuel make a 50 seater economical to fly?", the answer is a definite maybe...because of the involved variables.

For a major airline subcontracting small jet lift, the measure of that lift being "economical" is if the benefit it provides (revenue into the hub) is equal to or greater than its cost. A decrease in fuel price obviously lowers a 50 seater's CASM, but if revenue decreases more than the cost of fuel then you're right back to it being an uneconomical airframe because costs exceed revenues.

If there isn't enough revenue in a market to support the economics of multiple 50-seaters, there may be enough to support a reduced number flights on 70-120 seat aircraft because their economics are better. That reduced frequency on larger aircraft means fewer in-service 50 seaters.

And yes, I'm already scheming my way into airline management...