Professionals, please enlighten me. Would not a 24 - 90 2.8 or 28 - 90 2.8 be an "ideal" walk around lens. While I don't shoot full frame, I would imagine a lens of such focal length would be ideal for a general purpose lens given it's 2.8 (as opposed to f4) and is long enough to reach the "ideal" portrait focal length of 85/90mm.
I am not a professional, but I'll get a shot at answering.
If we are talking about a high quality FF lens, then a FF 24-90/F2.8 would need to weigh between 1kg and 1.2kg minimum. It follows that this would not be a "walkabout" lens, as such lense by definition needs to be smaller, lighter and unobtrusive. The industry has responded to this demand long time ago - in the film days. The best choices tend to be 24-105/F3.5-4.5 or 24-105/F4. They tend to weigh from 400g to 600g and are really a good walkabout compromise.
Note that, as the ISO sensitivity of the sensors increased enormously with modern sensors, the absolute need of having bright lens decreased. When all you had was a nominal 64 ISO film and no lens stabilisation then the difference between F2.8 and F4 might have been of a shot or no shot. Today you can get serviceable photo at ISO 12,800.
On the other hand the demand for better sharpness and contrast from full aperture opening and
across the whole image field and
for all focal length is now very strong - again driven by the excellent, high resolution sensors.
I am hoping that the rumored Sigma 24-105/F4 lens will be exactly like this - sharp and high contrast from F4 across the whole image and for all focal lengths. And if it had stabilisation then it would be my ideal lens.
By the way, your ideal focal length for portrait 85/90mm is not the same as my ideal focal length for portrait - 135mm. Again, this points to the fact that there is no such thing as an "ideal" lens. They are designed and manufactured mechano-optical devices and like all technical solutions each carries its own set of trade-offs. The art is to select a device that fits your real needs and budget.