If someone stows their bag as they should (wheels first) it is only a .5" decrease in size where it matters for most aircraft currently being used. This will be a 3.6% increase in capacity at most.

I fail to see the need for reducing the length measurement.

If the Boeing "space bins" weren't configured in sections like they currently are there MIGHT have been a potential win there. As they currently exist I don't think the change in height from 9" to 7.5" will result in any real extra capacity.

Better idea: have travelers store their bags properly. If it doesn't fit, check it. The capacity gains from enforcing this would be much more pronounced.