Does it really matter?
IMHO it does not matter what the aircraft or future aircraft CAN do, it is what is commercially viable. Forget ETOPS and different variants it's a case of is it worth it.
Ryanair thrive on been able to do deals with airports and region, i.e. In 5 years we can deliver 1 million pax if you give us this price. It usually is not destination reliant, so FR will try our (for example) a STN route, if that doesn't work try another and another, until they find the right combination of routes that work.
As soon as you limit your opportunities (such as transatlantic on a 738/MAX) you can't offer the same deals, as you no longer have 80+ options for routes, you have maybe 5 if your lucky.
So let's start with commercially viable rather than what's operationally possible before thinking that TA will happen anytime soon