The X53 was cut in May 2016. The X52 was introduced to cover the service to/from Exeter, whilst the X54 served the Poole - Weymouth part. If I recall correctly, this was done so that consistent times could be provided on the far reaches, as the timetable on the core part of the route sees journey times substantially increased in the summer season due to the holiday traffic.
That would have helped with duty lengths and avoiding journeys being dropped or cut short due to running out of legal driving hours. Plus the Exeter part wasn't the most well used part of the service, so when the decision was made, it was easy enough to drop entirely.
Likewise, changes have been made to the X54 to try and keep it sustainable, like withdrawing it at weekends over the winter, and diverting it to serve Lulworth. I doubt the latter would have been a sustainable idea when the X53 was running Exeter - Poole.