@Pin Head
This is ONLY a guess, but I think, "Something Else." Cargo fire suppression duration time can be enhanced -if necessary - to achieve ETOPS or extended ETOPS permissions, if/when a carrier wishes to spend the bucks. For cargo fire suppression it not a whole lot more than how much (volume/time) inert diplacement gas is carried. and how it is dispensed. (Of course there is more, but that's the biggie.) There ARE other concerns for ETOPS operations (737? you said??) beyond 60 minutes, but they are more than a bit complicated. Perhaps because of its relatively modest size, until recently the B737 was not a serious contender for extended ETOPS operations. Other considerations include an enhanced maintenance schedule and a proved history of reliability. As the most frequently sold, used and flown transport-class airplane in AV history, I'd guess that the properly equipped, individual B73 is not difficult to certify for extended ETOPS operations. Proof: Several carriers operate the B73 from the Left Coast to Hawaii, several times every day. So... while extended cargo fire suppression is one concern, there are others. The B73 series CAN be equipped for and can qualify for extended ETOPS operations.