I think 40 years is sorta that magic number. Ideally the 777 can last much longer due to the fact that there will be a large amount of spare parts available due to the production time and amount of aircraft. it is still a very real part of many airlines fleets but it is reaching the end especially a -200ER. The real problem is that United has to decide if hauling cargo is important to them. Because the only replacement for a 777-200 is an A350-900… or used 777s. The 757 and 767 will go before. That airframe is at end of life around the world. There are lots of spare parts but a 787-9 replaces a 767-300/400 and an A321 gets the domestic 757 and we have yet to see but based on Euro airlines flying the 321LR it does replace a 757-200 on an Atlantic crossings even without the XLR performance.