IMO, they wouldn't sell that well. While the initial surge would probably be decent people would quickly realize why they were never that popular to begin with and once their curiosity was satisfied they'd turn around and dump them on the used market.
Anyone who is serious about wanting a Mark IIC+ can easily get one. They're for sale all the time and the price isn't all that extravagant for a holy grail amplifier that was only made in limited quantities. Anyone who can't afford a IIC+ can easily buy an early Mark III and have the majority of the IIC+ sound, and as a bonus it's even more raw and aggressive... but no one wants a Mark III because of the compromises in it's design, which happen to be the same compromises that are in the IIC+. There's a reason you see guys like Hetfield and Petrucci using a TriAxis live... and it's not because they can't afford to gig real IIC+s.
On that note, other options for people who're chasing that sound include the Studio Preamp, Quad Preamp, and TriAxis preamp.
Long story short, there's a lot of options available for people who are serious about wanting that particular sound, even if they can't afford the real deal... and since I'm fairly certain that the majority of people whining about Mesa not offering a true reissue don't own any of this gear, I can only assume that they're not serious enough about wanting it to put their money where their mouth is.