That's not a fair comparison. The DS Lite and DSi XL were improved versions with better batteries. It's like saying that the PS3 Phat sucks because the PS2 Slim is much smaller. It just doesn't make sense.
Portables have always had improved versions over the years. Just look at the original GB, than the GBC, GBA, GBA SP, GBA Micro, DS, DS Lite, DSi, PSP, PSP Slim, PSP Go, and the list goes on. There will certainly be "improved" 3DS versions in the future.
The original DS had a 850 mAh battery which lasted about 6-10 hours. Not the best, but at least you could theoretically reach the double digits. The DS Lite had a 1000mAh and an improved backlit screen and lasted about 15-19 hours, which was a significant difference.
The 3DS has an 1300 mAh battery which lasts 3-5 hours, which is pathetic to say the least. As for 3DS XL, it has an 1750 mAh battery which lasts 3.5-6.5 hours, which is really not much of an improvement. If you want to see some real improvement, you must go with a battery like the Mugen pack, which is a whooping 5800 mAh and can improve the battery life to reach even 20 hours. Unfortunately, the Mugen pack is ridiculously expensive and costs just 100 bucks.
Unfortunately, for whatever reason, the 3DS is a digital vampire, sucking the life force out of those batteries before they know what hit them. Future improvements won't help much and since Nintendo is thinking that the duration currently is "fine", they won't bother.