Once the 10 car units are all in service, then they'll be commissioned. At this point they are excess stock, as they were ordered pre-covid when they were running more services than now, and further expansion was planned.
The 701 stock plan was designed/precured to replace the entire SW area suburban area with a single type of unit. Very good idea to me, before anyone slags me off! The 10 car plan as was in place under SWT was maxed out as far as could be achieved both financially (under DfT) and physically location constraint wise (ie; berthing capacity) at all relevant locations to the suburban area, including a host of siding lengthening's brought in to play in SWT days under my watch, but this was not sufficient to increase ALL suburban services to 10 car. It may have been partly a financial decision (?), but from a stock stabling capacity, the 10 car plan introduced under SWT was the maximum that could be accommodated on the network at that time (2014 ish).
The addition of 10 x 10 car roads at Feltham goes some way to providing additional 10 car capacity, but it still does NOT provide enough additional capacity to allow ALL suburban services to run as fixed 10 car trains day in, day out. Take that from me, it is not enough capacity.
And from an operational POV, on a daily basis, splitting 10 cars in to 2 x 5 cars (simply to stable in sub 10 car length sidings each night) and reform a few hours later for the next days start up, is in the land of fairies! It would never work. To give some perspective to that comment, departures from Wimbledon depot (and Clapham Yard also) for the a.m peak have long since been at their maximum, time interval wise, in so far as there are NO gaps in the timetable for any more trains to depart from those depots via any of the routes in question.
And here is the rub of it: to make additional shunts from one siding to another to reform 2 x 5 car units in to one x 10 car unit at either of those depots, would require a timetable slot (within the depot limits)
between other planned departure (times) from those depots, prior to its departure from the depot as a 10 car, and a timetable slot to match. Both of these scenarios were exhausted with the SWT 10 car plan back in 2014, when any such shunt moves were exhausted, in so far as to make any such shunts (within depot limits) would have the result of cancelling a main line departure from the depot, because the shunt move had taken its slot on the departure siding concerned.
I struggled to get this point across to certain very senior people in my time, but I believe a review of such matters after my time by Michael Holden, designed to understand the core reasons for a decline in reliability did address such matters.
PS: there are only so many routes out of Wimbledon or Clapham depots, and if any of those routes are occupied by units making shunt moves from one siding to another to reform from 5 to 10 car format, that route is then lost to any train departing that depot. End result being a loss of a pathway out of (the) depot to provide a train for service. This is not rocket science!