Wow, talk about missing the point.
Of course you can't make guarantees, but what you CAN do is quantify both the COSTS and the BENEFITS of doing something. Something with a low cost/benefit ratio is probably worth doing. Something with a high ratio is not. In your motorcycle example, we can look at history to make a determination: protected riders fare far better than unprotected riders, so the benefit is great. The cost is not absurd, so wearing protection makes sense.
So we can also apply this analysis to this problem. One the one hand, we have the current systems. What is their track record of dealing with change?
Went from running on 24, to 31, to 64 bit addressing hardware
Went from data stored on punch cards and tapes to online DASD
Went from standalone systems, to networked with SNA, to networked with TCP/IP
Went from green-screen UI only available to bank employees to mobile apps available to all customers
Went from all batch-processing to mostly on-line
Went from account information only being available via a mailed monthly statement to being available 24 hours a day, updated in real-time
Now, what is the supposed 'risk' with these systems? The pure FUD that 'there may be a change they won't be able to deal with'. Such as what, exactly?
On the other hand, we have the 'new and improved' systems. Exactly what is THEIR track record? Exactly WHAT change are the existing systems going to be unable to cope with, but the new systems will be able to cope with? Any examples at all?
It seems that the 'benefits' portion of the equation is sadly lacking (except for the FUD factor, of course)
Then there is the COST portion of the equation. The track record here is not good. There have been many public and spectacular failures doing what you suggest, all at enormous cost.
So what we see is a gigantic cost, and a benefit of 'maybe this' and 'maybe that'. When your cost portion is that large, your benefit portion better be larger, as in as close to a guarantee as you can get. You have not provided anything even close to that.