Obviously they are. They also done that to problematic B787 or A330, A320, B737NG or A380. But it's not major. It's a slight tweak over period of time. The major change gonna be the update from the -100 to the -200 which make the plane more attractive.
It would be naive of you to think no other aircrafts ever receive any tweak or improvements over 20 years of service.
The market growth are the reason why airplane with higher capacity like B737-800/-900ER or A321 selling like a hot cake. This day and age, there are 2 billions more people on earth, and billions of people who could afford to travel by air. In the past, most airlines would operate B737-300 instead of the -400 because the market is smaller. Now most airlines would operate B737-800/900 instead of B737-600/-700. This could be proven by looking at the number of sales for the 737-300 vs B737-400 to B737-700 vs B737-800.
So, yeah, the main reason why A321 getting really popular is because of the increase in air travel. And how there are emerging market for 180-220 pax segment on short flights.
You do know how to write a **** load of bollocks.