It strikes me that an airline now replaces a type that was replaced in other airlines years ago (B747-100 & -300). Actually, some airlines think about the replacement’s replacement.
Currently the market for pax-to-cargo conversions is good, but a 25 year airframe might be beyond the economical point. Assuming a lower average utilization of 3000 to 4000 hours a year we easily arrive at 75000 to 100000 hours, and I think B747 normally don’t get behind 120,000 hours.

I guess at least the -100 go directly to the bone yard.