5 hours London-Frankfurt is still materially slower door to door than flying, so the train would have to have other advantages (which it does, especially the German ICE trains in terms of comfort and wifi etc) to compensate for the additional time.

When the Eurostar first started on London-Paris with a 3h15 journey time, it wasn't a clear-cut decision, especially if you were travelling from somewhere other than near Waterloo station to somewhere other than near Gare du Nord. Now with a 2h15 journey time it's a quicker and easier journey for the vast majority of people (other than pax connecting to and from other flights, of course).

London-Amsterdam will probably already be a reasonable choice by train when they open the high speed line (anyone know when that's likely to happen? I understand there were technical problems) in the Netherlands, even if you need to change in Brussels and lose say half an hour as a result. As soon as direct trains run London-Amsterdam I'm sure that'll be a practical proposition.