Right now, 100% growth. But anything can change and those airplanes could easily become replacements.

It's currently 2 777, 15 787, and 28 B737. (source) There are also ~420 retirements in 2020.

However, it appears that UAL is very seriously planning on mainline growth. A new sim building is nearing completion in DEN and the next one after that is already planned.

Somebody in a cubicle at HQ is apparently expecting UAL to have more mainline pilots in the future, and the BOD has signed off on the $$$ for it.

All that said, I can't emphasize enough that the long term planning committee meets twice a day. We've all seen this movie before.