From “Quotations of Chairman David – the little red book of COTS procurement”:

The rider must stop to change horses on a long journey, even if he is in haste.

New product releases occur periodically, and products are sometimes discontinued. If a system makes use of many COTS components, maintaining it means upgrading its components regularly. The resultant overhead, whether measured in downtime, reinstallation costs, or anything else, is a necessary by-product of a COTS-based approach.

In addition, making small, incremental changes as the various COTS suppliers make their upgrades can prove to be much more economic than making very large changes at infrequent intervals.