Unlikely to be a 376 replacement - they're just about to be refurbished.
I wouldn’t put it past DfT to have some unpublished masterplan.
Something like move 376s to South London (Southern), release 377s that took over from old 455s, switch some 377s to 387 workings, reallocate 387s to somewhere like Oxford. But speculation is not really for this thread.
Returning to the tender announcement, the optional quantity (based on 20m length) suggests it’s not really decided if this order is directly replaces certain fleets, or also covers bit more than just the aging networker fleet. The option is not big enough to cover whole of 707 or 376 fleet.
There is a good chance that price vs maintenance cost of patching up existing fleet will be a major factor. Whilst a standardised fleet simplifies things, there are other potential possibilities (eg the old way of taking out a trailer coach, shortening a unit, limiting its speed then using them on branches). Similarly could you bodge some of the 2car and 4car networkers into 3car units, and restrict them to peak hour extras away from busy London sections. Variations that might be cost effective for a few years. The 465s are only 8 years older than some 375s, so if some can manage extra few years (even on restricted usage), they could be looked at with a 375 replacement order in 2030s instead.