IBM custom lockin, so that they were the only ones who could do component level repair, using the returned failed boards as the source. The only thing IBM did not realise was that the custom logic was so reliable that it would still be running a half century later in many cases. More than half of IBM's business comes from making sure that the new hardware is bug for bug compatible with old systems, even if it means writing extra code to make sure it is. Kind of like pulling the table out from under a dinner sitting and putting a new one in, while people are busy eating and drinking while seated there, without them even noticing.