Your 2nd para sums it up perfectly. The Kent timetable for services ex Cannon St run on a 20 minute cycle (off peak) and 22 minute cycle (peak). Note this doesn't mean a 3tph service to certain destinations, rather it is train paths that do roughly the same thing in the Metro area. E.g. 1702 Hastings, 1724 Ramsgate, 1745 (46) Folkestone, 1808 Ramsgate all have the same routing. Trains coming out of the TL core via London Bridge to Kent would have to fit into this cycle. But as we know, the TL trains would be in some form of half hourly cycle. So it won't work, at least not reliably and not without plenty of pathing time. Unless, of course, you completely recast the Kent Metro timetable to fit around a handful of Thameslink services. Hence why it would be somewhat easier from a timetabling perspective to keep TL services via London Bridge heading down the Sydenham corridor, and anything going to Kent to be routed via Bromley South.