I'm sorry, but that's just stupid. If T-Mobile abolished 3G and 4G, they'd literally lose most of their customers almost OVERNIGHT.
It would be equally stupid for Sprint. For one thing, 3G GSM (UMTS/HSPA) is basically CDMA2000-1xRTT with wider channels and some minor refinement tweaks. In fact, with a little software effort, you could even overlay CDMA voice/1xRTT users on top of the same channel used for HSPA in a fringe-rural area (or interior femtocell). 4 CDMA2000/IS95 users, equally spread between 4 channels overlapping a single HSPA uplink or downlink channel, would just look like a second user of the same mode to the other phone simultaneously using the same frequency.
Legacy GSM is kind of brutish with its demands, and EVDO is bitchy in its own way, but 3G GSM and CDMA voice/1xRTT data are "kissing cousins".
If they were ever forced to make an absurd decision between legacy GSM and HSPA, T-Mobile would be better off abolishing legacy GSM, because they'd lose far fewer customers.
By the same token, god forbid, if the FCC approves a merger between T-Mobile and Sprint, the best thing Sprint could do is quickly release radio-modem upgrades for higher-end Android phones and iPhones to allow Sprint phones to be Canada-like and use CDMA (from Sprint towers) for voice, but HSPA (from T-Mobile towers) for data when LTE isn't available. AFAIK, every high-end Sprint Android phone since the Galaxy S2 and Motorola Photon has been capable of HSPA. If you really want to split hairs, even the now-ancient original HTC Evo had latent HSPA capabilities (but no SIM card, though I think some guys in India eventually found a way to hand-solder a USIM meant for embedded use into it... score one for the good guys subverting carrier-enforced arbitrary obsolescence...)
Frankly, Sprint doesn't need MORE spectrum so much as it needs BETTER spectrum. Sprint already has more licensed RF spectrum than it fucking knows what to do with. It's just that 100% of it is 1900Mhz or above. Sprint could solve 100% of its coverage problems by throwing more towers at it. And even with more towers, what Sprint REALLY needs is more fiber backhaul. T-Mobile isn't sitting on ABUNDANT spectrum, but with the new spectrum they got from AT&T, they have enough to solve their own problems with more tower sites, too. See, that's the nice thing about CDMA (and HSPA) -- you can solve just about ANY capacity problem by simply throwing more fiber-connected towers at it. It's the killer feature of CDMA-based technologies.
In a rational & sane world, companies like Comcast & U-verse would start building agile picocells into their cable & VDSL2 modems, allocating a few mbps of bandwidth (independently of what's available to customers) to them, and allowing AT&T, Verizon, Sprint, and T-Mobile users to roam on them at some cost low enough to be a better deal for cell carriers than deploying more towers (obviously, the carriers would have to delegate a small chunk of spectrum to them, too). If every cable & (V)DSL(2) modem was ALSO a mini cell tower with a few hundred feet of range, even fringe-suburbia would become blanketed coverage zones within a few months. And the reduced penetration of 1700, 1900, and 2100MHz would become an advantage rather than a drawback.