Luckily nobody has to stand in a Voyager when commuting from Medway every day, as they don't run on that route...
In seriousness, coaches may provide some competition from Kent, but not all areas of London are as easily penetrated. Had motorways generally reached further into London from all areas as originally planned, I think perhaps competition would be stiffer. But try (especially) Surrey to London during the peak, and even if you have to stand, the train is a no-brainer in terms of usual timings. It is also worth pointing out that through travel can be a lot harder when changing coaches and that the door-to-door experience is not quite like a train - almost any railway station offers a better welcome than being dumped in the gutter of a road of a coach company's choosing, should you dare venture from a city centre coach station.
I'm sure we have previously had this debate several times and the matter of available timings did come up. I notice that nowhere on the quick "comparison" between coach and train on the King's Ferry website does it mention the speed or timetable, merely the frequency, which is not everything (especially given that you can't quite have as many stations as bus stops, or as many passengers on a coach as a train).
As for the statement that trains do not have a "tracking app" on the King's Ferry site... there are dozens of rail apps which can track trains, so I think their claims should sometimes be extended forth by means of bargepole just as much as any other transport operator.