Yes, but the TOC is not going to want to accept the units then have them sitting around for ever. As you say they'll have to pay for them and then there's storage, while still retaining the fleets being displaced. The impetuous has to be to get them into service as quickly as possible after acceptance.
I imagine in the protracted negotiations on the new contract DfT will be trying to screw down SWR on this as ultimately it'll be the Treasury paying for all this.
Totally agree, there are going to be different objectives
The train builder will want to be paid as early as possible
The leaseCo will want to charge user as soon as feasible
The TOC won’t want to pay for new and old units in parallel
The train builder will have an agreed delivery schedule, their software issues, and Covid will have varied this. It can only have slipped, not advanced, but can TOC delay it months if their drivers aren’t ready without paying for the trains.
SouthEastern might have contractual dates for 707s, so have to leave regardless of introduction of 701s
The leaseCo (of other stock 455s, 458s) might have specified minimum extension or notice periods. The TOC has its hands tied here as can’t leave itself with no stock, but would struggle to accommodate it all if there was big overlap.
But the commercial contracts are not in public domain, so can’t tell who is in the strongest position contractually re determining introducion timescales.