[phlummox]

The Sunk-cost fallacy is when you weight the value of something disproportionately because of effort or expense you've put into it.

But if you've got an OS that's certified for the work you're doing, and it's not costing you extra to work on that OS, then there's no fallacy - you're getting more value out of the cost of certification you've incurred, and shifting to some other OS would presumably require you to incur the expense of certification again.

That said, the skills needed to work with a legacy OS will tend to become rarer, so you ought to factor that into your calculations.