Properly designed and written scalable software can last indefinitely with little or no modification
The problem is that what may be "proper" today (thinking, say, 5-10 years out) may fail the test in 10-15 years. One has to design software under some constraints, usually defined by the problem boundary and thus it by definition cannot be infinitely scalable and flexible.
Also, the problem is the code ends up being maintained by programmers of varying levels of competency. Even if competency is not a problem, the varying styles inadvertently lead to spaghetti and inconsistent code.
The problem is that what may be "proper" today (thinking, say, 5-10 years out) may fail the test in 10-15 years. One has to design software under some constraints, usually defined by the problem boundary and thus it by definition cannot be infinitely scalable and flexible.
Also, the problem is the code ends up being maintained by programmers of varying levels of competency. Even if competency is not a problem, the varying styles inadvertently lead to spaghetti and inconsistent code.