I looked up it's life span. What makes this design so successful?
Its about the perfect kind of design for airlines to make money on. The thing about big jets is you have to fill them to make money because they cost a lot to operate. Its a lot easier filling a 180 seat Jet then a 350 seat one and the 737 has the range to handle "most" of the routes offered and used and they are cheaper to operate.
So they can fly "most" routes, are easy to fill, they are small enough to keep the airports happy cause they dont take up a lot of space and arent cumbersome to slow down airport movements. You see this when your on the tarmac and see these A380s, 747s, 777s, 330s, 340s, Lumber around the tarmac. They are big and slow to move around, and not just that, but when they take off or land they create a vortex in the air that prevents another airplane from going into that same airspace for minutes afterwards. The big Jets just plain slow up the dance and that costs the airports $$. Most of all the A380.
Plus the newer engines are so much better, fuel efficient, dependable...ect your looking at the new Max version having a range approaching 4,000 NM and an ETOPs rating that will probably allow it to fly transatlantic routes. Its going to be like a mini-757 with a far cheaper per mile cost for a passenger/seat.
Simply put the 737 is a money machine for everyone involved with using her. The A320 is real close too. Theres a vast market to sell them to, a huge amounts of routes to use them on, and they are easy to fill up with paying customers. Money Machines.