How many radios on the market today have built in CW decoders?
CW is a human mode, not a machine mode. Sure, a machine can send / receive it, but it was designed for humans to send / receive and a lot of it out there is still human sent. As such it can sometimes be difficult for a decoder to deal with. Yes, there are a lot of software and external boxes out there that can decode CW, and they work fairly well when the CW has good SNR and when it is machine sent or sent by an operator with a good fist.
But reduce the SNR or add an operator with inconsistent element length and CW decoders tend to choke, resulting in a lot of T, E, I, S, and H false / error decodes.
Maybe Icom did not want to deal with people complaining something was wrong with the radio every time it did not decode the CW correctly. Maybe Icom realized that you can't build in everything, and there are many existing solutions to decoding CW. Or maybe Icom realized that the best CW decoder is the Mk 1, Mod 0, human ear / brain combination, and a built in CW decoder was an option that a small segment of the users would actually use.
It would be nice to have every mode in the 8600, and as has been said, the hardware could probably support the software to do so. But putting the effort into the system to do "all the modes" (however that got defined, since that is a constantly moving target) would be significant, and really each individual mode may only benefit a small segment of users.
You want CW, Joe over there wants FT8, Mike wants Olivia, Teresa wants Hellschrieber, I want AT3104D 12 and 20 channel, etc, etc.
Instead of never getting it right, and never being able to make every user happy, it is just a lot easier to provide a line level out on the radio and let the user select from the myriad of software suites out there to decode his / her favorite mode with his / her preferred interface. Frankly, I was somewhat surprised they included FSK in the radio, and I assume they did so only because they had already done the work for another, preexisting, radio. Also, although I often receive FSK / RTTY, after the first time trying it out I have never used it again with the built in capability of my 8600, I always end up using external software for FSK / RTTY.
T!