Yeah, the cited depth limit of ~8.5km appears to be based on “fish use an osmolyte called TMAO to protect their proteins from pressure, and below 8.5km the concentration of TMAO required is so high that it inhibits cell function”. Apparently crustaceans get deeper by using five different osmolytes at once instead of relying on TMAO. (Info from helpful comment by mytailorisrich in this thread https://news.ycombinator.com/item?id=35403838)

So in theory fish could adapt to life at greater depths by “learning” to produce more than one osmolyte.