With changing technologies and cheaper, more energy efficient materials and designs, nothing will ever remain forever (isn't that true of all of us?). It will always eventually be cheaper and more cost efficient to replace rather than repair. No amount of marketing will ever make buggy whips sell in an automobile salesroom, so marketing isn't the issue. Product live cycle is what's important. Market it when its growning, but milk it for the best profits when its reached its peak.
At one point, thee were some who thought that a stripped down VMS might make a reasonable competitor for Windows, ... if DEC management has though that PCs would grow to be more than a toy.