We have legacy hardware that would cost $50k to $100k to replace.

Um, gee, that was my point.

How long do you think it will take you to write and debug a driver? How long does it need to run without rebooting or tweaking, etc? A week? A month?

Let's say you're paid $40k/year, and you need 6 months to develop a "production" driver. You might think your time is worth $40k/year, but to your company, it's more like $80k/year. You work for 6 months, and it costs them $40k.

What's the cost of ownership here? After the driver is developed, how much time will be spent keeping it updated with the latest requirements? 2 month/year? That's $10k/year. How about the hardware? What's its upkeep? What is that compared to new?

What could you sell this hardware for?

In the end, it may not make business sense to play with this, unless there is no cost in taking your (spare) time, and you just really want to write device drivers.

Unfortunately, hardware and software is not backwards and forwards compatible forever, or there would be a lot of museums with useful capacity.