Do mid-low end bike manufacturers only design their bikes to last 1000 miles?
have a look at this : https://en.wikipedia.org/wiki/Design_life

and this : https://en.wikipedia.org/wiki/Mean_time_between_failures

It is all a matter of statistic and probability of failures in the lifetime of the component.

This means that the component should have a normal life of about 1000 miles before a certain percentage of failure (can) start appearing on a number of components.

Another example, my washing machine has a design life of about 14 years [1]; before that the number of failure should be minimal according to the manufacturer, after 14 years, the number of failures should start to statistically increase.

It does not mean the component will fail after the design life, but that the probability of failure increase.

[1] http://homeguides.sfgate.com/expected-lifespans-washing-machines-67293.html