According to industry standards, airlines refreshes their cabin between 5 to 10 years to remain competitive in the market.
I asked one LTP worker, during PAL refurbishment of its 744 before, about cabin refurbishment and she was more or less on the spot.
Airlines usually change their business class cabin between 5-8 years while economy cabin between 6-12 years. Most airlines they worked with in the past changed their cabin altogether between 6-8 years old prior to their retirement.
One example she gave is Virgin Atlantic. When i asked why is this so, the answer was for this airline to remain competitive with their product offering. So they are updating the product to stay competitive.
With that timeline, two 777 is already out of range, both being 13 years old. If PAL intends to prolong its service, they just have to update its cabin, and they are good to go for another 6-8 years.
The issue with the 744 however is a long story. It wasn't supposed to be updated but replaced by 777 from 2006. Unfortunately, we were placed in category 2 by FAA, if you can remember that, preventing PAL from introducing new planes to the US and for which reason delaying its delivery. Because of its new cabin, it continue to fly until 2014, a whooping 22 years.