They likely knew continued training on code would have some amount of catastrophic forgetting on other stuff. They didn't throw away the old weights so probably not sunk cost fallacy going on, but since it is relatively new and they found out X% of API token spend was on coding agents (where X is huge), compared to what token spend distribution looked like on prior Geminis that couldn't code well, they probably didn't want the complexity and worse batching of having another model for it if the impacts weren't too large and decided they didn't weight coding enough initially and it is worth the tradeoffs.