Indeed. I am thinking of longer journeys. I was astonished at how quiet the trains often are.
That doesn't tally with my observations: For six months, I travelled daily on the 07:30 Birmingham - London Euston (the non-stop service), and 17:23 return. Both commonly ran with most or (Though admittedly not all) seats occupied. The 06:27 Manchester - London Euston via Birmingham, 08:10 departing New Street, is generally full and standing with an 11-car Pendolino between Wolverhampton and Coventry, and comfortably busy south of there. I'm not entirely sure how busy standard class gets on the 06:35 Manchester Piccadilly - London Euston (arriving 08:46) as I'm usually in first class, but the impression I get is that it loads well.
On topic, I think that seat reservations perform a valuable function that makes rail travel more attractive to many prospective passengers and provides reassurance that they will have a seat on the frequently overcrowded railway. They certainly shouldn't be abolished, but I do think that operators should keep at least 25% of standard class seating capacity unreserved, preferably in a large block clearly identified on the train's exterior, rather than Transpennine Express' and Crosscountry's approach on their 4 carriage Voyagers of almost every seat on the train being reserved.
Where unreserved seats are available elsewhere in the train when the full quota of reservable seats hasn't been utilised, then I think that the train diagrams displayed on the platform information screens at the likes of London Euston that identify the level of reserved seats per carriage are a very helpful indicator and should be used more widely.