For example one of my "rivals" in JFK has ~150 aircrafts on order!? And there's practically no slots left (at least when he gets them deliverd). Ok, he might replace some of his old planes but what is he going to do with the rest??
That competitor you speak of, Empire, is using one of the best strategies one can at slot-tight airports like JFK. Another one of your competitors, Pantheon, is doing the same thing.
And that is that, between the two of them, they've snatched up 50% of the slots at JFK through, primarily, the use of smaller planes -- Empire is using F100s, and Pantheon ATRs. They turn these planes 3-6+ times a day flying regional routes and snatch up slots. Leaving you, who went with larger planes, with few slots to work with. You got fewer planes over time than they did and your planes turn slower. So they've essentially starved you out of slots.
Then, they take all that money they're making and buy larger planes. Then, as the new planes arrive, they close routes that the small planes are flying and put larger planes on them flying longer, more profitable flights. An ATR may have flown to a place 3 times a day, using up 3 slots at JFK -- but they can put an A340 on that route once per day, then use the other 2 slots to fly somewhere else in the country. In this way, a single regional jet can give up enough slots for 2-4 heavies.
So you may think that JFK is out of slots for growth, but it really isn't. They're sitting on plenty
of slots for growth. Even if there was not a single slot left, they could easily grow their pax numbers 2-3 times what they are today simply by putting larger aircraft on the slots that they have today. A fleet of 50 ATRs can be sitting on enough slots flying short regional routes, to allow one to easily support a fleet of 200+ A330/340s on long-range domestic or international routes.
Now, as for the OP's question -- it is absolutely
worth it in my opinion to take out loans for new aircraft. Waiting to save up the cash can add months, even years
onto the production queues for aircraft. And since a single plane can generate $500-$1,500K per week
in gross profits, you're crazy not to. A delay of just 6 months can cost you $20 million dollars in lost opportunity costs by waiting for the cash rather than taking the loan. But, if you're going to do it, don't extend the payments out too long. I think if you can't afford to pay off the loan within 1 year then you shouldn't take it out in the first place.