The Discover 135 Sports may have found a 'sweet spot' in engine displacement that has been overlooked since?

Discover Sports had an interesting middle ground between smaller 99cc commuter bikes and larger 200cc sport oriented machines, with a 135cc engine size. This displacement struck a balance between performance and efficiency that was appealing to some riders for real world use. After the 135 Sports was discontinued, few manufacturers have ventured into this particular engine size, and rather have shifted towards the 125cc or 150cc+ categories. Discussion has arisen about whether the 135cc category struck the right balance for many riders in markets where fuel efficiency and price of purchase are as important as performance. The questions often turn to whether modern engineering could wring even more of this potential out of the displacement, or if regulatory and market forces make it impractical to revisit.