Här kommer lite om skilnaderna mellan Canons tillverkning och andra tillverkare att välja mellan och varför.
när utvecklingen går så fort som den gör
Design practice: This seems to vary company by company. For instance, Canon's sensors seem to be based on a pixel electronics configuration which is fixed over a 'generation', and applied to different size pixels. Obviously, pixels designed that way don't obey the scaling law. Sony, who presumably have greater design resources, seem to produce a large range of different pixel designs, and seem more likely (but not invariably) to produce a fresh design for a new size pixel. Their sensors are more likely to obey a scaling law. Panasonic seems to maintain two completely different pixel designs for similar size pixels. The side effect swamps any scaling effect.
Process limit: Semiconductor processes have geometry limits, go smaller and yields suffer, expense gets higher. Process limits aren't simple. How far down in scale you can go depends a lot on the detail design. The design attitude to process limits depends on your fab strategy. If you have a limited, in house fab capability, like Canon, then once pixels get small enough to affect yields the only approach available is to start playing with the design to eliminate those issues. This will tend to be an iterative process of redesign and yield characterisation, so is expensive. If you are a company with a larger variety of in-house fab, like Sony or Panasonic, then an option is to swap production to a line with finer geometry, and very likely the yield problems go. To do this you need to have the volume and variety of production which keeps all your lines busy.
If you are a fabless company, like, Nikon, Foveon, Fujifilm, Aptina or Omnivision then in principle you can buy your fab from a foundry with fine geometry processes available, although transferring design from one line to another is not straightforward. More likely you'll select a foundry on the basis of the demands of your design in the first place. Given that process limits aren't simple, most foundries work in terms of 'design rules' which are a conservative set of rules, if you follow you are pretty sure of a good yield. That isn't to say you cant get finer, but to do so will require the iterative design process discussed above.