As a commuter on the line there are too many trains trying to use a track that is at capacity. The slightest issue and the whole thing goes pear shaped, just like Heathrow on a foggy day.
The answer is to run fewer trains but to make sure they are all at maximum length (12 cars). Many of the off peak Gatwick Express services operate with 5 cars which is a complete waste of capacity.
There is no excuse for cancelling services at short notice, however even with the removed services there are still about 10 services an hour into Victoria for most of the day so there shouldn’t be a need to wait long.
Thameslink is a bit of a lottery due to the engineering work going on.