I believe that Jeremy Corbyn did state that he would accept a place in Kier Starmer's shadow cabinet prior to the leadership election result being declared.
If the above is correct, It would seem that place has not been offered to him which may prove to be a substantial mistake, for In that, he has also stated that he intends to remain as an MP for the foreseeable future
Corbyn I feel will now become a rallying figure for the left-wing of the movement who will now be looking for an anchor point. It should also be remembered that Corbyn along with Jon Lansman were instrumental in organising Momentum so as to become prominent in the Constituency Labour Parties from which position it now draws its strength.
In recent months Momentum has been recognised as an independent affiliation organisation to the Labour movement in its own right.
The above, I believe, may possibly prove to be the background for many in the wider Labour movement to call for a complete break with the present fragmented Parliamentary Labour Party in the future.
It may well be that there is at this point much goodwill towards Kier Starmer in his new role as leader. However, should there be any suspicion that this leadership does not intend to carry out the promises made to the trade unions and other affiliate organisations during Starmer's election campaign due to pressure from the right of the party, then above would surely be the scenario that swings into effect.