[fuzzfactor]

>Over 40–60 years they have accumulated layers of patches, updates and custom logic.

>enterprise overhauls often take 5–10 years, by which time the target architecture may already be obsolete.

That seems to answer the question right there.

There's not space in the room for a bigger elephant, do the math.

Once a replacement system has been proven to have outstanding performance in testing for 10 years, then you can be confident you may just be about 25% of the way there.

Why settle for less?

Be optimistic, 25% is excellent progress in such a short time :)