I think Gen 5 is where Pokemon's trend predominantly linear campaigns began. There are a lot of roadblocks that are only cleared out once you beat a Gym Leader and/or accomplish some other goal. I thought the game would ease up on the roadblocks after Castelia City, but it doesn't. Both the Driftveil Drawbridge and the Marvelous Bridge are closed. Do you think the game could have benefited from letting you access Driftveil early? I don't see the harm in it. In fact, I think that's where the next roadblock should have been; after Driftveil. The Plasma part of the campaign could be something that only triggers when Elesa is defeated. There's no reason Clay can't still get involved if you beat him before Elesa.

Every Pokemon game prevents you from advancing to certain areas prior to a particular task, but I think the older games pace their roadblocks much better. This is true even for Gen 5. The breathing room seems to get smaller and smaller with each successive Generation excluding Gen 4.