They were certainly intended as a long term solution: significant sums of money was spent converting/refurbishing (respectively) them specifically for the Caledonian Sleeper work.
Obviously plans can change with time, and whilst the 73/9s have been somewhat underwhelming on the sleeper, I don't see a viable replacement in the pipeline.
As far as I know, the 92s are meeting expectations - or am I missing something?
Ultimately GBRf are contracted to supply traction for the sleeper so it really is down to them what they supply. From a traction management perspective I suspect the only significant drawback of the current arrangement is the number of 66s it ties up.