What leads you to believe that the Thunderbolt was initially supposed to be a 3G only device? I'm thinking that if Verizon had any input that they would've told HTC that they should go ahead and throw a dual-core setup inside the TB since dual cores were already being pushed around that time. I know there was speculation that Thunderbolt owners got a few months of free wireless tethering because HTC couldn't get the Wifi hotspot to differentiate when the phone was surfing the internet and a wifi hotspot was being used, but that was just speculation.
There's really no telling what took place behind the scenes unless there's an official press release so all I can do is base my opinions on what I know took place. The release was a mess, the updates are still a mess, and the phone never really made a positive impression on me. The fact that I'm even considering going back to another HTC device proves that I'm willing to chalk the Thunderbolt up as a mulligan. I can forgive, but I'll never forget.
When the Thunderbolt first leaked as the Incredible HD in actual pictures (around August 2010) we were still looking at LTE phones not shipping until mid-late 2011. The Incredible HD was to be released around November 2010. There was no 4G LTE branding or LTE radio inside. BlackManX (Panda) even mentioned once that the release was pushed back to 2011 after the decision was made to go to LTE. Miami (on the Droid Forums) took a lot of flack that the Incredible HD didn't release in November 2010 as he said it was supposed to. His explanation was that it was going to be revamped with added goodies.
Whether you believe BMX & Miami or not, how often do we see devices in leaked photos (not renders, actual physical devices) almost 7 months before they're released? Normally you only see them around 2-3 months before release, which lines up with a release around November. Do a search of Incredible HD 2010 and you'll see what I'm talking about.
Switching to a dual core at that point would've been even worse. Look how long it took for a dual core to release. The first one on Verizon (LTE or not) was the DX2. But that was with a Tegra2. The first dual core Snapdragon (HTC's CPU supplier) phone to come to market was the EVO3D and Sensation around May 2011. Besides, Verizon wasn't worried so much with dual core, they wanted to push LTE for 2 reasons. First, the extra $100 on the LTE phones helps with the buildout of the 4G network. Second, moving more people to the LTE network means less on 3G, and with the i*hone launch coming they didn't want issues that AT&T had with their network.
As far as going only by what has happened or actually announced, if everyone went by that, every phone releases on schedule, and there are absolutley no issues pre-release. Maybe you're right and it didn't go that way, but when insiders (and numerous ones at that) are talking about it, it seems likely that it is true.
Here's a link on BGR from August 27th, 2010. After clicking the link, click the pic for the gallery of the first shots of the (then) Incredible HD.
http://www.bgr.com/2010/08/27/evo-ish-htc-device-verizon/
P.S. I hadn't noticed this before, but look at the back of the phone in those shots. On the Thunderbolt, the soft touch battery cover goes completely from one side to the other. But on this shot, the battery cover doesn't go all the way to one side of the phone. Never noticed that until today.