You'll note that I didn't say "no benefit", but "no benefit except" - and then give precisely the one and only benefit you say - the removal of Watford passengers from services to/from beyond the M25.
You have enough arguments against your proposal to rebut without having to make up ones that no one is making!
They are already less busy than the other trains in NW London, and are going to have a 25% increase in capacity by the end of the year at the latest.
The increase to 4tph (when the ongoing introduction of new stock allows - the timetable now has a gap for that 4th train to slot into) isn't driven by PIXC or even giving standees seats, but by the desire to have the magic 'turn-up-and-go' service level at South Hampstead, Kilburn High Road, Headstone Lane and Hatch End (and to a lesser extent stations outside the GLA boundary), ticking London Travelwatch's box. That they didn't bother considering this last time they ordered new trains for the Overground tells you that this really wasn't a pressing matter.
It also solves any future crowding problem - have a look at these 2041 crowding maps:
with only committed schemes,
with TfL's full strategy. There's literally no difference on the DC line trains, because this year's 25% increase in capacity means there's enough seats that fewer than 1 person per metre squared is standing anywhere on the line in the morning peak, even 22 years later!
I'd suggest that having near zero levels of am peak standees on trains that are designed to maximise standees (and thus capacity) 22 years from now with no upgrade planned beyond the imminent boost to 4tph, is very much proof of the trains being "pretty empty".
Why though - again, this is literally the pair of tracks with the least crowded trains in NW London, and frequency increases can be done relatively easily with the coming of new Bakerloo stock in the mid-2020s (which also increases capacity). The 2018 Mayor's Transport Strategy does propose further increase the frequency (beyond any 2020s increase) on the Bakerloo as an as-yet-uncommitted extension to Lewisham, a move that removes most of the low density standing the 2020s upgrade leaves on the line north of Paddington. We're certainly not talking, north of Queens Park, about anything in the 30tph ballpark though! Especially not at Wembley (as the additional trains from the BLE boost seem to turn at Stonebridge Park, not being particularly needed north of there).