...but there is a long operational cost associated with kicking everything the extra distance through to a siding facility. In London's case, due to lack of spare land until approaching the M25, this would need to be a very considerable distance for HS2 services terminating in passengers services.But they don't run continuously in a loop with passengers. There are still those that "terminate" at major station, but instead of parking/blocking a platform, they continue in the same direction to holding areas (for crew changes, cleaning etc) before they re-enter service so there is layover time built in, particularly on the longer routes. Of course, the shorter routes are very high frequency anyway so the timetable doesn't really matter, like the London underground. The points/changeovers are away from the main stations so cause less delay/disruption as they occur in less densely used areas of the network rather than at the mouth of a busy terminus station.
Eurostar? Last time I checked, St Pancras, Gare du Nord and Brussels Midi were all terminal platforms....It works with Eurostar which don't have terminus stations at each end but a loop at each end instead.
Berlin also seems to operate with several "main line" through stations rather than any terminus stations, with mostly pretty small/simple lines & stations.
Again different geography to London:
-The North-South axis through Hauptbahnhof (the new, low level bit) is generally the cross-Berlin services, connecting (For example) Hamburg to Leipzig, etc. Centres of demand on one side, with centres of demand on the other
-The East-West High level axis (the original Statdbahn) is different, given relatively little of Germany is to the East of Berlin (as it's not far to Poland). So here it makes sense to terminate trains....but lack of space to do so in Central Berlin (it being a narrow viaduct corridor) means through operation to Ostbahnhof to terminate and be serviced.