I've had different experiences. I worked at 2 places that threatened to relocate to the South, one to Atlanta, and one to Orlando. The first one was just a corporate fiat, as in "those NY guys are too expensive, close the office and move it all down here." The second was an active IDA poach by Florida. Both wound up not working...the first was because, as you said, the company would lose too many of their knowledge workers and they weren't confident they could find new ones. When they started bleeding people, they realized they might want to rethink it. The second was basically just called off because the company got a sweetheart deal from New York.
My experience has been that companies will try to move their less skilled work or work that is considered non-core (like IT) to cheaper locations first. If the target location gives them a good enough deal, they'll just move the whole company. This is probably just short-sighted MBA stuff...executives are largely shielded from whatever environment the lower infrastructure investment produces. Their kids will go to private schools, they'll live in the best areas, etc. so they don't see any of the problems and they also see a quick win. Even if you get to keep your salary and relocate, you'll either be up for a layoff right away or never have your salary increased by much again, until your compensation matches the local market. So, you have to take that into consideration; either you love the area you're moving to and make a decision to stay, or you'll have to pull up stakes again and move back.
When even the real estate agents running a relocation tour mention that your kids will need to go to private school to get an equivalent education to where you're coming from, you know you're going to be in for an interesting experience.