I see people post that on the internet uncritically but I really question that. The capacity lift from 77W to 779 is not that much since MTOW is still the same at 352 tons. 779 is 3 meters longer than 77W which means if J and W cabin stays the same size, airlines will be able to fit maybe 2 more rows of Y. That's 20 more seats at 10-abreast but with lower fuel burn due to newer engine and some aero improvements. Airlines don't even need to use the extra room for more economy seats... maybe you expand the J or W cabin so the seat count stays about the same. Basically, any airlines that already make money with 77W should be able to make money with 779.

A350-1000 has roughly the same seating capacity as 77W but much lower MTOW. This means airlines with significant belly cargo operations like BR will not be able to make as much money with A350 as 77W (or 779).

The reason you see the like of BR ordering A351 right now is because 779 is superduper delayed. It was supposed to enter service in 2021 and yet it hasn't even been certified and Boeing already said it is focused on getting 737-10 airborne first in 2024, which means 779 will be delayed to perhaps 2025. Airlines are losing faith in Boeing's ability to get things done. Years of cost cutting to please Wall St has reduced Boeing to a husk of former self and airlines are starting to see that the 737 Max fiasco and similar massive delays on 737-10, not to mention Boeing's inability to launch the MoM - the supposed 757/767 replacement that airlines have been waiting to see for nearly a decade, are not isolated incidents. Something is really wrong with Boeing that it cannot do the fundamental things airline manufacturer should do... to design and make an airplane that doesn't have serious flaws without ridiculous delays.

This is why it is interesting that BR has only order enough A351 to replace its earliest batch of 77W. Seems like they are hedging the bet a little. And why I mentioned that Boeing better wake the F up and get 779 flying ASAP.