To an extent yes, that's an issue. But these days TOCs are reluctant to do multiple splits and joins and by having longer fixed formation trains you reduce complexity, increase capacity, reduce build cost (2 cabs per 8 coaches is cheaper than 4 cabs) improve the driver environment, allow guard and catering staff access throughout the train, improve passenger circulation etc.
Having spare capacity in off peak times is not a huge penalty to pay for this. There will still be a place for 3 and four coach trains on quieter routes but busy commuter lines should be aiming to run 8 x 23m trains and ultimately they will be more financially viable and environmentally friendly that way as loadings increase.