I've been there. It was beautifully designed and perfectly suited to its job as headquarters for a telecom monopoly. With everything good and bad that implies. It's sad to see it fall into the hands of suburban mall developers.
The alternative is to knock it down and build something banal. At least the developers are trying to preserve it and preserving this type of building is really really tough. It's not a house, and it wasn't built with hand tools.
These buildings are vast. Every $1 per square foot of renovation is a lot of money. It is further complicated because the historic features tend to include long since discontinued manufactured items. Often these components were custom designed because the products were cutting edge, needed in large quantities, and the client's were such as to see the benefit and to afford them [e.g. Pyrex tubes at Wright's Johnson Building and the ceiling tiles in Bunshaft's Cigna Hedquarters].
Finally preservation is tough because these buildings were designed at a time when energy was cheap. Van der Rohe' Segram Building ran all the lights all night for architectural effect. All that glass which allows daylight in also creates a greenhouse. Technology can mitigate some of the effect but until we get a thermostat on the sun, the physics will remain the same.
Developers come in all stripes. Any neighborhood or district you love and enjoy was built by one [or more]. Sure, they might be lying scumbags and just paying lip service as a precursor to a Power Center in a sea of asphalt. But probably not.
That this developer is trying to preserve the building means that they are valuing history and aesthetics over the bottom line. They may not be successful. They may walk away. But to me, it sounds like they are instead dreaming unreasonable dreams, and that's the fabric of amazing places.
Actually, it wasn't the headquarters of AT&T and not even the headquarters of Bell Labs (a part of AT&T). The headquarters of Bell Labs was at another massive (though less beautiful) complex in Murray Hill, NJ
I always preferred Murry Hill to Holmdel. I found the architecture more interesting, and the copper roof to provide continuity in the history of the Labs.