A questioner asked on the Flickr Olympus E-System, "where had all the fast primes gone?" It turns out this person had purchased a Konica SLR film camera with a matching 50mm f/1.7 lens. 'Back in the day' that's what you usually purchased with any SLR film camera, an inexpensive, utilitarian 50mm that ranged in speed (and cost) from f/2 up to f/1.4. There were some that were faster, up to f/1, but at that point you were talking serious glass, physical size, and money. Lots of money.
The heyday of the 1970s had far more SLR manufacturers, and thus more prime, or single focal length lenses. Every manufacture spent the 1960's learning how to manufacture a lens that provided quite reasonably affordable image quality. The typical SLR 'kit' consisted of the body and a 50mm (or normal) lens, between f/2 to f/1.4, typically around f/1.8. So, with all the manufacturers in the market and with the standard lens being 50mm, market forces combined with economies of scale drove the price of the general 50mm down to $100 or less (in 1970 dollars). The cost of a 50mm was usually 1/5th to 1/3rd the total cost of the kit.
Two other popular focal lengths that also benefited from economies of scale were 35mm f/2.8 and 135mm f/2.8. Prices were more competitive for those lenses due to the fact that second tier manufactures also made lenses in those focal lengths and speeds. It was thus very easy to build up a film SLR system out of a body and three lenses that would cover the majority of the shooting situations that most photographers were interested in.
The 1970's also saw the beginning of the zoom lens. In the beginning they had, at best, 2:1 zoom ranges (35mm to 70mm was quite popular, as was 100mm to 200mm). Unfortunately they were either cheap and bad, or very expensive and very big. It was far cheaper, and the results far better, to use primes.
It continued that way until the mid-80's, when companies like Vivitar and Tamron began to manufacture reasonably good zooms at affordable prices. I purchased a Tamron SP 70mm-210mm one-touch constant f/3.5 BBAR (Tamron II Adaptal mount) for what I would have spent on an equivalent Minolta or Olympus 200mm prime at the same speed. It sported a macro capability (1:2.6 magnification) at all focal lengths. Finally, to add a little icing to the cake, all lettering was engraved instead of silk-screened. It is a sweet lens with great performance for its price.
With the 70-210mm and a 35-70mm I had the ability to continuously cover every focal length from 35mm to 200mm. I shot primarily in normal light (and still do) so that both zooms were quite fast enough for what I wanted, and they were well-behaved wide open. I discovered, like so many others, that I didn't need to carry a bag full of primes around any more. Through the 1980s and into the 1990s, fixed-focal length lenses became the niche lens, while the zooms came to dominate, including as the default lens in standard SLR kit. The last film camera I bought, a Nikon N90 in 1989, came with a Nikkor 35-70mm f/3.3-4.5 AF zoom.
I purchased my first DSLR in 2006, the Olympus E-300 two-lens kit with a 14-45mm f/3.5-5.6 zoom lens and a 40-150mm f/3.5-5.6 zoom lens. The two digital zooms covered a 35mm film equivalent of 28mm to 300mm, a good deal more than the 35mm to 200mm I had with film.
Since picking up the E-300 I've purchased two more Olympus DSLRs, along with some additional zoom and fixed focal-length lenses. I wanted a greater focal length range, which was satisfied with the 9-18mm, 12-60mm, and 50-200mm. This gives me an effective continuous focal length equivalent of 18mm to 400mm. All with three zoom lenses. The two fixed lenses are now my specialty lenses.
There's no going back to fast cheap primes unless you buy an adapter and purchase film primes on eBay. If you want modern fast primes for digital use then expect to pay a goodly price for them. The two I currently own, the ZD 50mm f/2 and the Sigma 30mm f/1.4, were both around $450 when I purchased them new. And that's pushing the upper affordability bound, even factoring in inflation between 1970 and now. In the two photos above, I compare the physical size of the two fast primes with the E-P2's 14-42mm f/3.5-5.6 kit lens. The kit lens give a 35mm equivalent of 28-82mm.
What has happened over the last 40 years is that the primes and the zooms have changed places; the primes have gone from utility to specialty tool while the zooms have switched from specialty tool to must-have utility lens. There is no getting around the fact that today's zooms will take the place of any number of fixed focal-length primes, for a fraction of the combined cost of the primes the zoom replaces. And don't forget the size differential. That little M.Zuiko in those photos is a fraction of the size and weight of either prime. It's fun to shoot with those primes, but if I had to pick only one, I'd always go with the M.Zuiko because of it's diminutive size, light weight, and the fact that in practice it produces photographs of high image quality that are, in practice, indistinguishable from photos produced with either prime.
Why have prime lenses? For their narrow depth of field wide open. Right now bokeh is all the rage in the various photography fora. These lenses provide a reasonably inexpensive way to experience the joys of out-of-focus areas. I've seen far too many expensive fast primes costing thousands of dollars (Canon's an Nikon's 50mm f/1.2, or Lieca's even faster f/1 and the $11,000 f/0.95). After watching the remaining camera manufacturers struggle to produce we have now (continuous focal lengths, light weight, superb contrast and sharpness) over the last four decades, I thought I'd never see the day where out-of-focus blur has become the new sharpness.