Old is Gold: Here is an outlier in the world of ever changing advertising campaigns

This is a classical case – defies all trends, all classical wisdom, and if seen as a ‘wave’, would just sound a death knell to all advertising agencies.
I saw on TV yesterday an old old ad – one for Nirma ‘beauty’ soap – I think the commercial is atleast 10 years old, if not older, and has aired unaltered off and on throughout that time. I viewed it objectively – and, honestly, did not feel fatigue (which I had half expected), nor did I feel that I knew all I wanted to about the soap – I just felt a bit of nostalgia, and ofcourse, a renewed sense of admiration for the sheer beauty of the celebrity model in the ad….
And I started thinking – if this old – really old – ad is leveraging awareness of the brand; as well as evoking liking for it thanks to a) the nostalgia element and b) the model, then WHY are so many brands working at changing advertising every single month, if not day, and spending millions of dollars that could either beef up their bottom line, or be ploughed back as other investment? Look at the US fast food industry for example. Apparently there are anywhere between 500 - 1000 new commercials released every month from the top national QSR chains alone.
Of course, I know the answers – there is freshness/ competitive pre-emption/ changing hooks or differentiators/ topicality/ repositioning – all of that. I am sure there is a lot of truth in all these arguments.
But, for the companies who in the face of the recession are saying – should we increase advertising expenditure because consumers need greater persuasion to buy; or should we reduce advertising expenditure to protect our bottom lines; if they were to follow Nirma’s path, atleast they save the money spent on creative execution!