Actually, DeWalt's refusal to make an adapter for the old 18V line is simply marketing. True, most of the "pure" LiIon tools like the 20V, the M18 Milwaukees, Bosch 18V, etc have some form of electronics in the tool that communicate back and forth to the battery pack for temp and usage extremes safety, but there's no reason an "adapter" can't be made to interface the new with the old. Why? It already exists, though in a packaged form. The LiIon 18V XRP stem pack for the legacy 18V tools has this circuit built in. The XRP LiIon pack has 10 18650 cells in a 2 cells in parallel/5 cells (pairs) in series (2P5S) arrangement identical to the 20V Max "large" battery pack with circuitry built to fit inside the legacy "stem" which originally contained one of the original spec NiCd cells for cell protection. The other difference is the need for the special "universal" yellow LiIon/NiMH/NiCd chargers for charging. The charging process has to be strictly monitored and the old two/three-terminal NiCd interface can't hack it. A magnet inside the LiIon 18V pack beside the stem trips a switch in the charger and changes the function of the third terminal normally used for the cell temp sensor used in the NiCd packs. This allows the increased communication to happen between the pack and the charger. In simpler terms, it's far easier to operate the tools from a "converted" pack then it is to charge them, since charging of ANY LiIon pack needs strict controls of initial current and terminal voltage or there will be fireworks.
It is very possible to make a fairly simple adapter to operate the tools from the 20V packs with just a current limiter to prevent too much current pulled from the pack and a voltage cutoff to shut down the tool when the battery is discharged too far. The additional terminals on the 20V packs help with this information for cell protection. Charging is another matter, but the solution is simple. You can come up with a circuit that interfaces to the various cell protection circuits in the 20V battery and triggers the needed universal DeWalt charger, but this delves deeper into their proprietary technologies. The adapter can be made rather "dumb", only allowing the tool to be operated from the pack, in order to charge, you have to use the modern charger for 20V. Simple solution there, minimal $ outlay for the user for the charger. You needed a different charger for the LiIon XRP packs anyway, if you never moved up from NiCd, you'd still need the battery and charger in LiIon.
Those who have LiIon 18V Makita tools will notice that most of the tools have only two big terminals connecting to the battery and nothing else. Makita tool electronics are rather unsophisticated for many of the simpler tools, there's only a low voltage cutout built into the speed control. Many of the tools don't even have an overload circuit; stall the tool and keep your finger on the trigger and the magic smoke escapes. It would be easier to adapt a Makita 18V pack to the DeWalt 18V tools than would the 20V Max line. You'd still need a Makita charger.
In short, DeWalt could very easily make a nice, slim adaptor to slide onto the new packs and plug into the old tools. The problem here is it doesn't sell new tools, and that's what they are in business to do. If some third party were to make something like it for the greater tool marketplace, DeWalt will send their team of yellow-and-black suited lawyers to shut you down, citing some breach of patents and IP in their designs. Keeping all those 20 year old tools that have been working perfectly and continue to do so for another 20 years as long as there are batteries available is something none of these manufacturers want. Cordless is their cash cow, and that's why you see all the feature and tech push there. Corded tools **** the hind teat, because once they are sold, there's no more reason for the user to interface with the company with that tool unless it needs replacement. Batteries conveniently die in a few years, and unless you want paperweights, most people buy batteries. In some cases it's cheaper to buy a new tool with the batteries, charger and case than it is to buy the two batteries alone, or some small amount more for the kit vs. batteries. Why is that? Guess.