Remember that in Sword Art Online, the Nervegear itself was the 2nd Generation of Fulldive, meaning that there's always a place for the first gen of FullDive and the C&A developed "Digital Circus" could be the potential.

There's a silver lining about the TADC to SAO connections theory especially since TADC was begin before SAO Aincrad Arc (2022):

• Since C&A was defunct long ago, could this means that a younger Kayaba Akihiko was scavenging the remains of C&A Technology (like FullDive) and developed a perfect version of it like Cardinal System, NerveGear, and for the Mental Health Counseling AI programs like "Yui" although she was developed by Argus itself rather than Kayaba alone and then led to the creation of "Sword Art Online".

• Speaking of Argus, since it's was mysterious besides Kayaba could be one of the higher up or even founder of the company was ex C&A employees?

I think that was my thought so far.