And if it's the pumps going dry, why is that all of a sudden not a problem in the air? There we fly them until the LOW PRESS flickers and then turn them off.
You are not going to let them run dry, since you switch them off in all situations were there is a risk that they will do so:
1) On ground with less than 453 kg
2) At takeoff with less than 2300 kg.
3) In cruise when the low pressure light illuminates
4) In descend with less than 1400 kg.
With the modded pump replace 2 and 4 with
2) At takeoff they auto shut off if they run dry momentarily
4) In descend they auto shut off if they run dry momentarily
Additionally, with the modded pumps you have the protection as well if you forget in 1) and 3).
The problem in climb and descend with the
unmodded pump is the master caution logic. If only one low pressure light illuminates you don't get a master caution light. Hence, in climb and descend you are not getting any warning if only one pump is running dry (only on recall). Due to the pitch attitude, one pump may run dry for a prolonged period, with no master caution because the other pump may still be covered.
With the modded pump you rely on the auto shut off feature to shut the one pump off that may be running dry.
For the 3) cruise, the pitch attitude is such that the pumps will be able to reach all the fuel
and they run dry almost simultaneously. Normally one low pressure light flickers for a short while (a minute tops) followed by the other along with a master caution. Boeing must have found it acceptable that one pump is running dry in this situation because it will be for a very short period of time.
Disclaimer
The above comprises my theory on the matter and should not be taken as a fact.