Well, the 07 and 7550 are both slightly warm-ish / neutral phones. The 7550 is drier with less bass / a more peaky and grainy vocal section (which works for some tracks, esp. female vocals) than the GR07, but with a much more expansive soundstage and zero isolation as per design.
This isn't really true though. The HD700 was heavily dogged here and is only a so-so seller, probably far less popular than Sennheiser expected. Other products have been given the same treatment and have ended up in a similar position. The Shure 1840, Ultrasone Edition 10, Grado PS1000, and likely Denon D600 come to mind. Hype trains were set in motion (aside from the D600 due to aesthetics) and then derailed and now they are poor sellers, the Edition 10 in particular selling used for much less than original price.
While I wasn't really following the whole thread, the impression I got was that the HD700 is almost universally regarded as flawed - there's little controversy about Sennheiser gumming it up. If Sennheiser made a HD750 I would imagine the excitement over it would reach explosive levels, considering how much everyone hoped the HD700 would take the best elements from the HD650 and HD800. A spectacular failure sets the stage for a spectacular comeback, etc. We don't watch movies about sports teams that repeatedly play competently.
While I'm not suggestion that underlying technical ability plays no part in a headphone's popularity, sometimes it does feel as though there are animal spirits.