Ultimately the experience argument is pretty sterile because the role of a pilot is so different to that of a steward/ess.
In the highly technical piloting profession experience is critical and is reflected in the minimum experience levels required for command. These in turn are dictated by insurers who understand that pilots in charge of big bits of metal have to have extensive experience of their trade to operate as safely as possible. More experience is generally regarded as a good thing.
In the highly 'people focussed' cabin crew trade experience may or may not be an issue. Some people are excellent after only a few weeks on the job, and maintain that standard for the rest of their career; others are attracted by the lifestyle and never really become dedicated to their passengers. They will always do the minimum amount of work and, essentially, will never improve with experience. That's human nature.
BA has a large number of long serving cabin crew, and the above is as true of them as anyone. Some are excellent, a lot are mediocre, and some are truly terrible. Experience will have improved some, disillusioned others and had no effect on a few. It is no guide to how good they are at their job.