3G data transfer support on the iPod touch 5G would require:
a) More efficient 3G chips.
b) Much better battery technology.
c) A complete redesign for the antenna.
Have you not seen mifi modems? Tiny. Access 3G data just fine and then also blast it back out as a wifi signal. Last for a good number of hours. Retail for around $100. Do they not use efficient 3G chips (and have for the last few years)? Do they require much better battery technology (I bet the battery in the iPod Touch now is better)?
I'll grant you "c", but a redesign is often fundamental to hopping from one generation to the next. If Apple's reasoning is connected to making their iCloud as accessible on iPods as it will be on 3G-connected iDevices, they'll have to redesign the iPod to build in 3G tech anyway. And there are some rumors of a slightly bigger model (bigger screen), which creates the opportunity for added space and a bigger battery.
Yes, 3G would be possible on the 5G iPod touch, but at which cost?
If you look to the iPad as an example, Apple chose to offer with & without (3G) models. Why not do the same here? If so, that could also be a general example of "with 3G" pricing relative to current iPod Touch pricing (noting what happened in pricing when they went from gen 1 iPad to the gen 2 "redesign").
Similarly, again, look to the pricing of mifi modems. Strip away their parts that wouldn't be needed (for example the outer case since the iPod's outer case would cover that). If one can buy a whole, stand-alone 3G data device for $100 retail (sometimes one sees $79 and even $49 retail), I would think Apple could build in the underlying tech for not much more than the implied costs involved in making whole mifi modems. Then, those that don't care could buy the "without 3G" iPod and those that might want it could buy the "with 3G" option... just like on the iPad.
On the iPad "with 3G" adds $130 to the price. If an Ipod Touch 5G was available with a "with 3G" option for those interested (and data options that matched the no-contract options of the iPad), I, for one, would opt for the extra $130 to include the 3G flexibility. I'm not saying it should be forced into EVERY iPod but in my own case, +$130 for a 3G option on a gen 5 would get my money in the next month or two; no 3G and the money most likely stays in my pocket. If I am not a one-man market, there's probably a nice little pocket of opportunity there for Apple... those that don't want the total cost of thousands for an iPhone but could use the occasional 3G access via an iPad-like "use it when you want" 3G plan on a tiny device that fits in one's pocket.
I'm hoping Apple is thinking "iCloud on everything" so they include a 3G option (hopefully choosing to make it available for less than +$130 to entice more people to have iCloud-anywhere access); but I bet the big selling point (probably unstated) would be the on-demand, "cheap" data-only plan hooked to a portable iDevice which seems like it would be perfect for all those "text anytime" teenagers with parents that don't want to pay the full voice+data toll that comes with an iPhone. In my own case, I'm not that teenager, but I do have an occasional need for 3G access (certainly far from enough to ever buy an iPhone). In the past, I've spent my money on other company's technology to buy mifi modems etc. If Apple would rather have that kind of money instead, they build that mifi right into the next iPod Touch.
There are an awful lot of teenagers hungry for an iDevice but not the AT&T or Verizon monthly toll. They want to text but not much voice, though AT&T & Verizon forces the voice plan cost to get the data plan. If the parents balk, the kid ends up with Android or less. If Android, they accumulate Android apps while they are teenage poor. When they can pay up for their own "real phone", they would be faced with a choice of iPhone 7 or 8 and re-buying a bunch of their favorite apps again or sticking to the platform where their app investment already works. Thus, even though this penny-pinching market might not seem ideal for the short-term, the availability of app platforms does make it a longer-term upside market. Apple should get it rather than giving it to Android or others. A few years from now, those poor teenagers are prime candidates to pay up for the next-next-next gen iPhone. Already being locked into iDevice apps would practically insure that they buy Apple in the future.