Modern business practise has moved on rather - as you say yourself these 'rules' are part of the 'archaic world'.
Most modern business advice is that it is acceptable to use names and it is not grossly informal. Likewise, 'Sir/Madam' is 100% acceptable these days.
I wouldn't say that Debrett's is a guide to the modern business world.
Etiquette is about making rules to enable people to be polite to each other and have good manners. Most people now consider It is more polite to use someone's name if you know it. Having said that, there are still circumstances where I would also use 'Dear Sir'.
This is an issue upon which there is much debate!
So, do what you feel is appropriate for the cirumstances.