There’s three factors for development of a 500 f/4 G.
1. The technical bits of optical construction. Supertelephotos take years to develop and manufacture, and they’ll often be sold for a period of about a decade before they see an update. It’s non-trivial to build these, so you generally want to get it right before you sell them, and any production issues tend to be a setback. Witness the general non-availability of the Nikon 500 f/4 that was recently introduced for about a year after announcement. It just takes time to produce precision optics. Would people have preferred that Sony stayed silent about the development of the lens (and get slammed for doing so), as opposed to giving the hint (PMA 2009) and more info that the lens is coming? Of course, to some, those things are in effect the same because they can’t go out and buy it.
This is probably the least of the worries for the lens, but are worked out with basically time, money, and resources. I don’t think the question is if they can make a 500 f/4, it’s more if they would. Which leads to…
2. Economic bits. In the case of our system, one has to take a hard look at what kind of users would buy a 500 f/4. It’s a fraction of a fractional user base at large. How many do you estimate you’ll be able to sell over the course of those ten years? Two thousand? Would be really interesting to know exactly how many 400 f/4.5s and 600 f/4s Minolta sold over the years. If the economic case for starting production for it in 2010 wasn’t there, and 2011 looked like a better year (given upcoming sister products like faster camera bodies and a somewhat recovering economy), wouldn’t you take the hit and delay it a year which will, in the end, be a very short time period in overall lens sales? Yes, you might lose someone like Mark who just can’t wait anymore, but a few lenses here or there aren’t going to make or break the production of this lens. It’s going to be whether or not you can sell enough of a gross to actually make the development and production money worthwhile as well as being able to front the manufacturing process to begin with.
These, incidentally, are the same reasons why we’re probably not going to see T/S lenses for a long time, if ever. They’re an even narrower market than supertelephotos.
I think people underestimate how much the professional guys who shoot sports sort of subsidize the well-heeled amateur who wants a 500 f/4 or 600 f/4. Canon knows that the NFL, World Cup, baseball, etc shooters with major publications, papers, and leagues are going to spring for new supertelephotos as soon as they’re released. This gives a built-in market for several thousand telephoto lenses, which basically pays for their R&D, and the guy who wants to take a few trips every year to shoot wildlife in National Parks is gravy for them, and how they get their profit.
Who does Sony (and Minolta before them) have? Some well-heeled amateurs and very, very few professionals, probably none of which who shoot highly visible sports. It’s basically the same group of guys who’ve been in the system it for a very long time now, many of which already have some super telephoto options. Plus, you will have natural turnover of users, some leaving, some going, and the market is, unfortunately, probably inelastic. You’re not just competing with Canon and Nikon at that point, but also against yourself. Fortunately, the time is right for a new supertelephoto lens, given the existing options have been well out of production and are noticeably inferior in terms of AF (screw drive) to the competition.
This is a bit of a chicken and the egg type of problem, but ultimately if Sony wants to be taken seriously it has to offer the 500 f/4.
3. A suitable camera to use the lens with. While one can use a 500 f/4 with an a700 or a900, neither of these cameras were known as high performing cameras in the terms that people who care about sports and wildlife think about. The framerates are merely good, not excellent, and the AF is sub-par compared to the competition for this market. Without a suitable high spec, high performance body, the lens’ users will continue to curse the shot that they missed. Even if they don’t need a high framerate, they do need powerful, accurate AF. Of course, given what Sony has done on the entry spec (and cheap) a55, you have to wonder what they could do with a body at twice the price. If the a77 is able to do a full nine/ten frames per second with complete control (as opposed to the partial control in the a55), that’d be a pretty good start. The a55 is already pushing ~6.5 frames with full control. The body has to be comparable to a 7D or D300 in many (but not all) ways, otherwise, it’s a non-starter and will just continue the slide.
Of course, whether or not Sony can get people on board the train at that level of user is the question. If Sony had simply refreshed the a700, given it a few new things with moderately improved AF, it probably would not have sold very well and would go over like a lead balloon (see Olympus E-3 and E-5). Pentax might sell you a K-5, but they’re in no better situation than Sony and in some cases a worse one. Pentax is content to living in a market niche. Olympus users are even more screwed than either camp. If Sony tries to be a true third player, it will take a lot of time, money, and anguish, and would probably ultimately fail.
Sony is the Airbus that’s trying to disrupt the Boeing/Douglas duopoly. The only real way they can do that is a combination of two factors – 1, selling something that neither guy has (A320/SLT or on-sensor phase detect with global shutter). 2. one of the other guys needs to be horribly mismanged or lose a lot of business to reasons not related to you.