So what's wrong with extending the course? Frankly I'd rather pay for something that was all inclusive, rather than breaking it up into 50 sub courses.
Most people wouldnt. 90%+ of divers want to take 2-3 days out of a holiday to learn to dive in gentle conditions on holiday and do that whilst remaining in budget. Its for this reason all the main agencies tailor the courses for that.
Follow-the-leader resort diving doesnt need more than 3 days and most people only want that sort of diving.
As for cold water, its the "local conditions" thing again. All of that is perfectly possible on the main syllabus.