Back when I was a computer programmer building custom applications for clients, I had to deal with a similar situation - clients asking for my source code. It seems clients had a valid concern about their ability to continue developments and improvements to an important application after my sudden death or going out-of-business.
Since programmers rely heavily on income from on-going support, bug-fixes and feature enhancements, giving the client the source code would be a bad idea economically. However, I was not unsympathetic to their concerns about long-term serviceability of the product I developed for them.
The solution was to deposit the source code into an "escrow" account held by an attorney. Each program revision would generate a new escrow deposit. The contents of the escrow would only be released to the client upon my death or business closure. The cost of maintaining the escrow was paid by the client on an annual basis. Their concerns were addressed, yet I was assured of continued income during the life-cycle of the product. It worked for both of us.
Does such as escrow for your RAW files seem like an option here? As long as the photographer is alive, reprints can be ordered. Upon his (her) death, the RAW files can be delivered to the client. The potential for continued income for the photographer is preserved, as well as quality control and copyright issues, and the client is guaranteed continued access to the original photography indefinitely.
One business model is to lock the customer in, so that the cost of using someone else is high (i.e you retain the RAW files, the source code, etc.).
Another business model is to keep the customer happy so that he chooses to do business with you. Give him the RAW files, the source code, etc., and still retain his business.
Part of my marketing strategy is to make sure my customers know that I want to keep them as customers for a very long time. I make sure that they know my goal is for them to stay with me because they
want to, not because they
have to. My customers know they are free to leave, and by and large they choose to stay. I have happy customers.
When I was developing custom software, this strategy worked very well. Generally, it cost the customer less to have me maintain the product as I was familiar with it. The customer always felt safe and secure as they knew that they were using me by choice, and could switch vendors should they be unhappy with the quality of my work.
Occasionally, I have a customer that chooses to leave. I am always nice and polite, and make sure they have all of the material that they have paid for.
More often than not, they end up unhappy with the new vender, and they end up coming back to me.
.
But this is just my marketing strategy. It is not the only marketing strategy. You have to find a strategy that works for you. Locking customers in by making it difficult to leave is a traditional, and generally successful strategy for mass market sales. Apple does not license their operating system or their digital rights management software. If you spend money on content for your iPhone (apps or movies), you must walk away from that investment to switch to a different brand of phone. For people who don't care what phone they have, this helps keep them in the Apple camp.