What an engine manufacturer looks for is a long runway program with lots of aircraft in the offing and a reduced need to compete with one or more competitors. For example, GE declined to compete for an engine on the 757 program for that reason and the fact they didn't have an engine to fit the aircraft at the time. GE did decide to partner with Pratt on the A-380, but that minimized the risk of the program not being successful (long runway) in the end. The A-350 will be successful if the combination aircraft/RR engine proves to be what was advertised. There is still work to be done on the engine to make that happen...