You might find higher-reliability flash or EEPROM memories
in the "HiRel" market space along with some newer types
like chalcogenide memories (CRAM) and ferroelectrics
(FRAM). However you are certain not to like the price.
Speculating on acceleration factors is probably not any
good, unless you have the manufacturer's reports for the
product you plan to use you can't draw useful conclusions.
Design of the memory element and its programming cycle
are tradeoffs involving multiple dimensions (die cost, retention
time, write cycle endurance (these last two play off against
each other pretty directly, application supply and temperature
ranges, yield.
If you picked a gentler programming cycle and were willing to
accept a worse low-retention-time bit-population-tail you
might well get to a million write cycles. But are you willing
to do your own reliability work? Probably not.