GMC is an add on brand for GM dealers that don't have the ability to sell Chevy trucks. That's all and no more. Their product portfolio only needs to offer the main truck lines for what will probably become Buick dealer mates.
GMC can be profitable even after the marketing and cost to alter the style from Chevy. Trucks carry a higher premium to absorb this and have been doing so. There really isn't a reason to drop them.

There was a time when Caddy wasn't cool before the last Gen CTS and then things turned around with product first. While the brand was never completely irrelevant it was down for a long time. No doubt it was helped by the Escalade phenomenon but it still needed the current product portfolio to get back in the game.
The same can happen for Buick if they stick to a few great cars with appealing style. The 2010 LaCrosse is the first Buick since the GN/Turbo Regal that I've had any interest in. It can happen.

As for the graphs; They show a decline in numbers which correlates to the dip in vehicles sold industry wide. When total vehicle sales drop 5-6 million vehicles in less than 2 years there will be a dip in each brands' vehicles sold.
Market share per brand also mirrors the dip in GM market share while in the gas to credit crunch and then into bailout/bankruptcy. I don't think these are problems specific to just Caddy, GMC and Buick.