Notice BA are using A350s on their route from Heathrow to San Diego. This was previously served by 747s and 777s.
Not sure if specific aircraft are preferred for different routes and appreciate some airports may be better suited for some larger aircraft but having travelled Heathrow to San Diego quite a few times between 2015 and 2019 it was always 777 or 747 whereas LA and San Francisco sometimes used A380.
I know the runway at San Diego was considered a bit short and on one occasion pilot of 747 did advise us the engines would go into reverse thrust quite soon on touchdown so maybe these are all reasons.
You can’t really mix the 777X and the 777, it’s different aircraft.
You could say that really about any aircraft where new models or variants are manufactured., Boeing’s web site lists it on the same page when referring to 777-9 777-300ER and the freighter.
And the 777 is not a developing aircraft.
This could be interpretation, developing, evolving, improving.