A good question leading to a better insight to the difference in "no maintenance" and "failure" in the context of this article. The latter will be determined by the design and show up in MTBF (calculated and demonstrated in life tests/field). Former suggests that the reliability/mtbf is not affected by leaving the units installed without on-going maintenance during this period. Taken to its conclusion then the quality of the on-site installation is an important consideration all other things being equal.