It's a long distance premier InterCity route (other than the Birminghams). You don't want to be sending everything back out within ten minutes with all the coffee cups and crisp packets still all over the floor and no time to stock the buffet, first class kitchen and trolley, there needs to be time to properly service the trains, which means you need a dwell at the London end of at least 20 minutes, ideally 30 (and add a bit on so it leaves again on the correct clockface time). It isn't Southern, Southeastern or SWR, and cannot be compared with those.
You also don't want to be interworking the branches too much as it causes delays to cascade across the whole operation. (I'm aware the nonsensical Avanti working agreements don't allow two trips the same in the same day for crews, but as I recall most crews only do one round trip a day anyway, it's only when you work a Birmingham that a second one is viable).
For a properly maintained, resilient InterCity service you really need about 0.8 to 1 platforms for 1tph, or thereabouts, minimum. Look at how the four platforms of EMR's bit of St Pancras struggled with 4tph, and I'm sure are now even worse with six. (That station was designed for the 2tph operated on the Midland in BR days).
I suppose you could tunnel out some reversing sidings, but would that be much cheaper than just finishing Euston?