Is it really providing the best experience for premium-paying passengers to subject them to "keep walking this is first class" for potentially up to half an hour at the start of the journey?
First-world problem maybe, but nothing would ever improve if we took that attitude to everything.
How do you tell if the person walking past is a premium-paying passenger looking for their carriage / seat or a hoi-polloi passenger?
Latecomers (i.e. often everyone at Euston!) walking through the train at the start of a journey don't annoy me, but sometimes there behaviour does. Two of my pet dislikes are people who'll enter an empty carriage - on a train that's already moving, so it's not going to suddenly fill - and insist on spending 10 minutes faffing around to find their seat, rather than just choosing somewhere empty. I've seen a couple do this at Euston then spend the next 15 minutes moaning about having seats with no window. Yes, those seats are awful, so just move somewhere else. (I moved instead, just to get out of earshot of them)
The other is the "next stop Euston, start packing everything up and stand up" routine so many do on Virgin West Coast. It's bad enough when they do it at Watford - and walk through to the front of the train, because those 30 seconds must be so vital to them - but sometimes it starts not long after Milton Keynes, which is really silly. I don't recall it being as bad on other routes, at least not the Great Western.