It wasn't a huge flop, or anything, but it was expected to do A LOT better than it actually did, and contrary to your statements, it didn't outsell Guitar Hero 5 by a "considerable margin" (the difference was less than 100,000 total across all three platforms from the data I could find, which isn't the huge gap you make it sound like).
If there was going to be a Beatles: Rock Band 2, it would've happened already. They wouldn't be waiting until after they discontinued DLC for the main Rock Band series, effectively killing off what little shred of the genre still existed.
That's not to say there is zero chance of it happening, but you're only fooling yourself if you think it's likely.
I have a feeling that it's very likely Harmonix are working on The Flaming Lips: Rock Band. My proof is the fact they haven't said they're not.
Witticus: If your husband takes you to Gay Days at Disney and starts making out with Goofy and Pluto, you might have a problem.
Can't wait for the announcement of Fall Out Boy: Rock Band.
Fall Out Boy 
Elton John 
The Red Jumpsuit Apparatus 
Mayday Parade 
Sara Bareilles 
"killing off what little shred of the genre" is a necessary evil to further music video game development, oversaturation is well documented and the whole format needs to be revamped for the NextGen consoles, and from what I read from Harmonix representatives is alot of comments suggesting that they remain a committed and dedicated group (that's still hiring). The details are foggy and where there's smoke, there's fire. Don't expect a whole lot of information on new technology (for obvious reasons) but if, in fact, music video games are on the verge of a new groundbreaking format to coincide with NextGen consoles, I'd put money on both Harmonix AND The Beatles as "likely" participants...:-)
Yeah, a new Beatles game is real likely, given that it's been how long since any Beatles DLC? I've heard of grasping at straws, but you're wearing the straws down to little bent twigs. Let it go already.
A peek into my RB dreamlist:
Simon and Garfunkel: "A Hazy Shade of Winter"
The Spinners: "Spaceballs"