Where are they going if they don't? You are adding more trains into 13/14 and where do you send them afterwards?
I believe the Piccadilly reversing siding is presently unused? However I believe there are still pathing issues, and unlike terminating in a platform you need to check the train is empty before taking it off there which would waste paths in 13/14.
If a two year full closure is the only way to get platform lengthening, then I think accepting the platform length limit is the better option.
Is it, though? Post electrification TPE will likely run 200m EMUs, including on the excessively busy Scottish services. Are you happy for it to become a station that only handles Northern services, with peak Blackpools which could go to 8.350 in time no longer calling either? It would be cut back to be served only as often as near-useless Deansgate, pretty much.
I'm not clear why a 2 year closure is necessary, though, given that it would effectively be able to be built in halves and connected up given the layout they are going with. Some closures would be necessary, and the Warrington-Manchester stopping service need to be replaced by buses due to the loss of the turnback once 5 was disconnected*, as well as the Southport going to Vic instead (which many support anyway), but a full 2 year closure?
* I do wonder if, if the concourse isn't to be extended, keeping 5 might make sense just as somewhere to dump a failed unit or one you don't have staff for, a bit like 1 is used now.