The transistor ratio per TF disparity doesn't seem too crippling, particularly since there would have been choices made for the consumer Navi's target market that could have been adjusted to favor compute if it were similarly dedicated. That aside there could be other issues like the risk of involving Navi in products aimed at the datacenter and HPC clients.Like I said, it's harder to distinguish what's "new architecture" and what not, as so much DNA of the "old architecture" gets always carried over. It still doesn't change the fact that in some contextes RDNA is still referred as GCN.
Like no-X said, it is worse for compute in terms of transistor budget. CDNA looks to be GCN1.4.x (since it's gfx9xx (908 IIRC)), biggest changes to 1.4.1 (Vega 20) should be removing some graphics related blocks
Navi at this time has some bugs that would make it less compelling for compute in particular (memory addressing mode bugs, LDS bugs, etc.), which may have contributed along with AMD's limited software support for why Navi's compute performance and implementation have been poor or subject to some significant errors.
Driver changes for Arcturus may point to some significant changes, as in some form of vector unit that is architecturally distinct from the existing SIMDs, potentially targeting large vectors/matrices with multiple levels of precision/accumulation.
There were also possible placeholders or existing graphics elements mentioned even if the graphics command processor was specifically missing. It could be that some amount of geometry and pixel capability remains, or it was less disruptive to the architecture or driver base to leave them as-is than to totally remove them.