I'm going to give a hearty two thumbs down to the production lines closing after 20 years change as it is simply game breaking for large airlines. This will have significant impacts across all game worlds in a very negative way and reduce the amount of fleet diversity, strategic choices and long world playability especially given the uptake rate limit of new planes. Planes fall out of favor and cease production because they are no longer economically viable (for the most part) in the real world. In our simulated world, we don't follow the same economic model - specifically related to fleet commonality, which is a force that makes the simulated economy vastly different from the real economy.
Let's look at a few cases where this will be game breaking -
The "final" 3 types of "large" planes in game - A32x, 737NG and MD90/717. The first two are fine, but the MD90/717 combination left production in the early 2000's after a very short production run. For game worlds that run until 2035, this means a MD90/717 flyer will not be able to get new planes for the last 10 game years of the world and potentially have to switch types. This will lead to all MD flying airlines to likely reconsider and get in the huge line of the A32x and 737NG.
The 757 - Seeing a resurgence with cargo capabilities, it will also not be available in the final 10 years of the game world.
There's a number of more examples, but if the fleet transition/4th type penalty is not modified, then this change breaks the game for large airlines that use types that were put out of production before the end of their economic life.