I'll give my experience as an example of what's being talked about in regards to "rapid expansion".
The current incarnation of my airline is the fifth. I had to re-enter the game as the first four attempts failed miserably. Not because of what I'd done, necessarily. I've done quite well in previous versions of this game - look at TJA - I have KMSP by about 65%. And that would be the second time I've managed that level of success in a full game.
There was a noted issue with low load factors with startups that was eventually fixed, but I'm convinced that it crippled my ability to grow my early attempts at an airline, which cost me valuable time within that first 18 months. I'll admit that I tried to expand a bit too quickly (which I'd been able to manage in the previous version) the first few times, but even slowing that down a bit in my later attempts, I wasn't able to make sufficient profit in order to sustain operations. Only once that load factor issue was fixed did my load factors reach (appropriate) levels where I was able to sustain a profit and grow the current version of the airline.
By this time, there was substantial competition - one airline basically following my usual model (regional to national to international) and another content with being a small regional, basically flying prop-commuter flights and doing well at it, at least at first. There were a few others, but they had/have not posed substantial competition (this is the first time I've ever seen KMSP with 4-5 airlines HQ'd there).
Then, an airline based at Chicago-O'Hare opened up shop at KMSP. Given their size at the time, I can only surmise that they were able to take advantage of either that airport's major demand domestically and internationally and/or the "C/F starting strategy" before the game engine was updated to block that particular strategy. In either case, me and my closest competitor were almost immediately dropped to 2nd and 3rd place at KMSP.
At this point, two airlines that had based at KMSP are gone, including that #3 competitor, the small regional is under major pressure and I can no longer engage in substantial growth as this new airline has effectively eaten up nearly all viable slots at the airport. Given the price pressure I'm experiencing now, I can only assume that an attempt is underway to squeeze me out of the market entirely, leading to a fifth bankruptcy. They may succeed, which is going to irk me, having just started QualiTeam up in this world.
The limitation on "no top 20 airports" allows airlines that setup at those airports to effectively setup "fortress hubs" at such airports. If an airline at a "top 20" is able to eliminate the competition based at their HQ, they have the ability to rake in massive profits which they can use to order massive numbers of aircraft (which dramatically smaller airlines' ability to expand outside of the used market) and drop in airports just below the "top 20" and effectively crush any airlines based there. They can then repeat the cycle. There is no opportunity for other, established airlines to compete at those fortress hubs, as they're blocked from doing so. Any new player going into such airports don't stand much of a chance - two aircraft with the startup money aren't going to make much of an impact against an airline owning a percentage of its aircraft, a monthly PAX count in the millions and a company value in the billions.
The top 20 restriction needs to go away to permit better (and more realistic) competition. And if it doesn't, then it at least needs to be reworded to specify "top 20 global airports" - having seen the screenshots prior to the launch of this world, it was my (incorrect) assumption that as a U.S. based airline, I wouldn't be able to expand upward and that airlines based at the top 20 in the U.S. wouldn't be able to expand downward into my airport. Had this been clearer to me, my strategy would have been different.
I'm also wondering about the time limit. If, for whatever reason, an airline can afford to expand before the 18 months is up, they should be allowed to do so. This restriction does not exist in the real world and I'm not sure time is the right measure of when an airline should be "ready" to add hubs. Number of aircraft makes sense as one measure, particularly if the "C/F strategy" no longer works - not being entirely familiar with the airport side of this business, I would imagine that an airport's administration would not be willing to commit the resources in permitting/establishing a hub without the guarantee of a minimum of business. I'm wondering if perhaps a given percentage of their HQ's (or prior hubs) market share might be a better way to go. Nothing terrible (I'm thinking 10-20%, perhaps even lower).
I'd be curious to see the effects of some of the other changes proposed...I'd hope that they're thoroughly tested before implementation. I'd be happy to volunteer as a tester if needed (I did it for six years at a Fortune 100 company).