Well you can sit down and do the maths; you mention Cobol, which you can actually map to the cost and availability of developers. The cost of that technological choice just keeps growing and growing. You can compare that to the cost of converting it to e.g. Java (and multiply that cost by 2-5x because it's very hard to make an accurate guess).

This goes for all technological choices. The cost is not static, but varies over time depending on market forces.