To make a positive posting about why the "board" receiver doesn't exist.
.. . First, I consider my PC. While my very first PC had a bunch of
cards, my current one has none except the RAM. The video adapter,
Ethernet capability, sound system, modem, serial, parallel, and USB
ports are all on a single board, built in.

Why? Simple -- it's cheaper.

I worked at Tektronix for many years. During that time, Tek made both
portable scopes and laboratory scopes, the latter having a mainframe and
plug in modules. For a given configuration with the same features, the
lab scope was always considerably more expensive than the equivalent
portable. Why? Well, the lab scope was always overdesigned for any
particular job. The bandwidth of the interface had to handle the highest
frequency plugin. The power supplies had to handle the highest current
plugins, in any combination -- enough current at 5 volts for a digital
analyzer plugin, enough higher supply voltage current for a spectrum
analyzer plugin, and so forth. There had to be enough connector pins and
supporting circuitry to handle all possible controls on all possible
plugins. No single configuration ever used more than a fraction of the
built-in mainframe capability. While the portable scope's stages could
have optimal gain, in the lab scopes, the signals always had to be
normalized to the levels specified for the interface. This often
required an extra stage or two for each of the signals being passed
(vertical, Z axis, horizontal, and many controls). Power supplies had to
be decoupled in each plugin at the interface. And finally, good quality,
reliable connectors are much more difficult to find, much more expensive
to buy than you'd think -- and even so, they can easily be the least
reliable components in the system.

Then there's the problem of trying to predict what would be developed in
the future when you design the mainframe, so you can build in the
necessary interface circuitry. And every new plugin (I've designed them)
has to be compatible with every tweak and trick used by all plugins in
the past which it might be used with.

The fact is that hams, for sure, wouldn't pay all the extra money a well
designed plug in system would cost. Of course, I might be wrong --
anyone who thinks so (one particular person comes to mind) should get
busy designing and developing one. Perhaps there's a fortune to be made.
Certainly there's a market for a much simpler plug in system with much
less versatility than the oscilloscope system I described, as a few
manufacturers have shown. The question is, how far can this be taken
before the market dries up due to the increased cost?

Roy Lewallen, W7EL