Michael,
If you set AEL to Toggle on a Sony (instead of Hold, which is the default), it will do exactly that.
AF/MF Toggle will also do the same thing (it just toggles the camera to Manual focus). My a99 behaves exactly like how you describe Pentax and Fuji when both AEL and AF/MF buttons are set to Toggle. Your old a900 would also behave the same way. It may not do that out of the box (because people expect those things to be holds instead of toggles since the Minolta days), but once you set those two things to toggle, it will not move. Presuming that you set one of your C buttons to an AEL or AF/MF function toggles, the a7 series would also behave the same.
Should those be holds or toggles by default? Any given user can probably argue the pros and cons either way, that's why they're a preference. As a UX person who has managed some very complex software, these are not unfamiliar problems. Cameras are used by varying degrees of user skill and even the highly experienced users have different ways of interacting with the camera. I would say that a hold (instead of a toggle) is a better default because a toggle is a lot riskier. Press a toggle once and the camera stays locked, and if you're inattentive or press it by accident you have the potential to ruin more shots. Some people really want holds, so give them the option to do it if they know what they're doing.
On a tangent, here's a good question without a right answer - should the cameras have several C-buttons, or should one or two be sacrificed for a silkscreened function that can nevertheless be reassigned? I personally vote for "have a preset function that is clearly silkscreened but can be reassigned if needed." But I can also see the downside - C buttons set the expectation that "hey, this could be custom assigned, better ask before using." If you hand someone the camera, they won't get mad if the button does something they don't expect. It's a C button, after all! Sony used to be very rigid about this in the past - the only button that could be changed arbitrarily was the single C button on the upper level SLRs. But now, you can basically customize any button on the back of the camera. The a7 series has a total of eleven customizable buttons: The wheel's spin, C1-4, the three cardinal directions, the center button, AEL/AFL. From a purity sense, the prior way was better, but for actual users, I'm guessing the "let me reassign my buttons" wins out in the end. The latter has the downside of everybody's camera being different. I'm sure if Michael handed me his a7r he would not have it configured in the same way I would, even though we shoot similar subjects. It's a downside, for sure, but us being able to configure each others' cameras the way we like them probably outweighs a more limited (but very standardized) function set.
One thing I don't really like is the AEL/AF-MF physical switch on the back of the a7 series. It really should be an AF mode (Continuous/auto/single style from the a700/900/77) switch, with the button being the AF/MF control full time and AEL going to where the C3 button is. Fortunately, you can set C3 to be AEL, but I would prefer it to be the default. That way you could set the AEL mode of the two-position switch to be something more needed, like AF Mode or AF Point selection.
