fdr:
not to be too simplistic, but as the calendar pages go by, there will come a point in time when going down to the local apple store and buying a bucket full of iphones and plugging them into the system will be cheaper than the cost of grounding a multi billion dollar program over the concern of a battery mass. The NiCd battery has been running for a fair old time, the guys in the hangar will be pretty familiar with them, and there are enough ex douglas guys there to string some more cabling as they were fond of doing.
Right now, the simple solution is a least risk solution.
As I posted a week or two ago, there's nothing 'simple' about the 'solution' of changing to a different battery technology - and weight is the least of your issues.
First, the charging and monitoring systems and software would have to be redesigned and recertified.
Second, where would you PUT the damn things? Any alternative battery technology will not have the same energy density as lithium. Not only will batteries of equivalent capacity be heavier, they'll be bulkier. Much bulkier. They won't fit in the space available, the space Boeing carved out for lithium batteries. Mk 1 posted on this a few posts back.
They would have to reengineer the structure of the EE bay - or even carve a separate battery compartment out of the hold spaces. Now you have structural engineering changes to design, test, certify, put into the production process, and retrofit to the units already delivered. And do this while synchronized swimming with all those subcontractors you've parceled the project out to...
Now do you see why I said this airplane isn't going anywhere soon?
(the alternative - finding the root of the problem and proving lithium safe enough to use, all over again, could be even harder...)