I've been frustrated by this problem ever since the sticker shock of pricing a new battery and charger for my first cordless - a 12V Porter Cable "Magnequench". For the price of 2 new batteries and charger you can buy a whole new drill kit. What is the logic of this? I also know that the basic cells that make up these packs can be bought in quantities for less than $1. So an 18V battery pack that sells for $70 -$90 has only $15 worth of cells.
The big problem is every tool has a different interface. Some brands have changed their own interface 3 or more times. If there was a universal interface, you would see 18V packs for under $30. But there isn't, so if you want to market a universal cordless tool power source you will probably need at least a hundred special adapters just for the professional grade tools alone. A couple years ago, I built a house for a guy who owns a plastics company. He was very interested in my ideas. I was looking at various options including variable output voltages and belt packs with coil cords. He even wanted to do a back-pack compressor for air tools. But as we discussed it, I knew it was hopeless. Tooling would run about $5000 per mold. Each interface connector will probably need at least 2 molds plus electrical contacts. I could quickly see that your talking about a million dollars just for tooling. Then there's the legal question of selling battery products that fit someone else's tools. Add at least $50K just for a preliminary patent search and legal opinion. I don't think you could sell enough batteries to ever justify a multi-million dollar gamble. Then along comes Ridgid willing to guaruntee batteries for life. I don't care if I do have to send the tool back for replacement. As long as Ridgid is in business, I'll never buy another battery.
And finally, one comment on the cord vs. cordless issue. Source of power is not the only consideration. Cordless driver drills offer much more. I have yet to see anyone but Sears offer a variable torque clutch. And no company that I know puts a brake on a corded drill. You just can't drive screws safely with a corded drill because of the rotational momentum. I used a Milwaukee screw gun on a subfloor once for an experiment. But OSB has too variable a density to set the depth reliably. I've even tried an air drill. It was really sweet but you need a good sized compressor to keep up - a little pancake unit won't do. As good and cordless drills are today I have no use for corded models except for hammer drills and Hole-Hawgs. If anyone's not being responsive to the customer, I say it's the corded drill makers.