They also evolved from the original Airline Control Program (ACP), which was written in the 1960's, at which point assembly was pretty much the only way to get

They also evolved from the original Airline Control Program (ACP), which was written in the 1960's, at which point assembly was pretty much the only way to get the level of performance required. I think there was some incidental "vendor lock-in", given that IBM was so dominant in the mainframe space for so long, but I think the more relevant principle is "conceptual lock-in", kind of like why banks still rely on COBOL. There was such a large investment in assembly that rewrites into higher level languages -- and we did consider them -- were prohibitively expensive.