It kinda makes sense though. The expansion from boundary scan to debug is just extending the scan chain to all the flip flops in the random logic too, not just the one's controlling the pads.
That’s a bit of a stretch though... jtag for ice/debug at that point just becomes another serial bus interface with a high level protocol controller.. you’re not literally extending any scan chain in most cases. The jtag port was coopted for this purpose simply for the pragmatic reason that it’s a good use of pin real estate.. nothing more.
Those high level protocol controllers came later to speed the process up and allow debug without exposing ridiculous amounts of IP to the debugger vendor. Originally it was just access to the full internal scan chain.
It’s a little more to it than that... I think you are severely understating the necessity of “speeding up the process.” IP concerns aside, an internal scan chain may still not give you features such as practical monitor mode debugging with any useful level performance. The point is, debugging in general is better served by dedicated hardware, at that point putting it on the jtag bus vs any other serial bus is more a basic pragmatic issue.
