On the Bristol to Birmingham example we now have the somewhat perverse situation whereby XC have been allowed to reduce the size of the trains from 7-8 coaches to 4-5, causing massive overcrowding on most, and then as a result use the concept of supply and demand to increase price. At first glance it makes the position reasonable - 'Our trains are filled to bursting point, we use price to moderate demand on these services' but then when you look at why they are so full...
When Virgin CrossCountry were running services between Bristol and Birmingham prior to the introduction of the Voyager fleet the service wasn't the clockface half hourly that it is now. There were large parts of the day when the service was only hourly at best.
Also it was extremely rare to see 8 coaches on a pre-voyager CrossCountry service between Bristol and Birmingham. HSTs were all 2+7. The loco-hauled, whilst also most often 7 coaches, (occasionally 6, very rarely 8) were predominantly Mk2.
Overall, Voyagers matched or slightly increased capacity by dint of the half hourly service. They also reduced journey times. I'm no fan of the Voyagers, but if the pre-2002 rolling stock and timetable was still in use today the service would be just as crowded. I'd disagree that this particular corridor is massively overcrowded. I've never had trouble getting a seat on a Voyager between Birmingham and Bristol, in either direction and at different times of the day. Between York and Leeds/Sheffield and Birmingham to Derby is where there is real crowding issues on CrossCountry.
The hard truth is that rail passenger numbers have increased massively in the last 10 years. Rolling stock provision has not kept up, but then I doubt very much when the Voyagers were ordered and the increased frequency was planned that anyone was predicting a 42% increase in passenger journeys between 2001-2011.