They can sell it. What they would lose in selling it secondhand is a lot less than what it would cost to rent the computer few times. In fact, renting the dive computer few times, will cost almost the same as what it would cost to buy an entry level dive computer.
Now for the bit about not diving anymore, in my experience over 3 decades of active teaching at all levels, the reasons for people quiting diving have largely to do with inadequate training and their competency in the water leading people to fear diving and not owning equipment making it expensive and highly inconvenient to go diving and hence they eventually stop diving. One other reason for people to stop diving is the unavailability of local diving make it more expensive and inconvenient to have to travel to great distances to dive. The OP here LIVES in the Bonaire of Europe that is a hot bed for excellent diving satisfying all type of interests in diving with a population that is especially fond of the sea throughout their history for the past few thousand years.
This is one of the biggest myths one hears in diving. You are assuming that the diver actually has a wide range of dive computer to rent of any type or model or brand of dive computer, regulator or BC for them to try them all to make an informed decision. In reality, dive centers frequently have the lowest priced dive computers and dive equipment available to rent and they often have only one model of each that is the lowest model in the manufacturer's product line. They also often don't have the latest version of dive computers or other dive equipment and usually only have ones that are few years old that don't represent the current models. If one finds an exception, a dive center with more current dive equipment to rent, this dive center will most likely have only one brand and one model hence the diver won't have the chance to try "different" models. It is a myth that doesn't related to reality of the market but sounds "significant."
We are scuba divers not free divers, we depend on equipment to facilitate for us to dive and to enhance our dive experience in safer conditions. I rely on my dive computer just like I rely on my regulator or my fins. I know what to do if and when my fins or regulator don't work as they should but It is most difficult and unenjoyable to dive without them, a dive computer is no exception.
No benefit at all. Unless you have lots of money and you can afford to buy everything under the sun, you won't be able to rent many different dive computers. If you were able to rent that many different dive computers, you would have spent enough money renting to buy a decent dive computer and probably a back up computer too.
I don't see the relationship at all. The divers needs to know and understand the principles of decompression theory and the concepts behind it but not necessarily "dive tables." At least two of well known international dive agencies have eliminated teaching dive tables and teach the basics of decompression theory and emphasize the use of dive computers for their students. Most concepts of dive tables used in diving are also used with dive computers and they aren't limited to dive tables. Using tables puts a heavy penalty in dive times without any apparent benefit at all. Manual dive tables may make the individual diver more susceptible to make errors in figuring out dive tables on a rock and rolling boat while struggling with motion sickness and other distractions. Compare this scenario with a diver who understands the basics of decompression theory and knows how to use their own dive computer and doesn't have to try to figure out the letter groups or RNT or MDT or SIT or TNT while puking doing that, there is simply no comparison in safety, convenience, and available dive time at all (more dive time with dive computers in their dive vacation vs. using dive tables)!!
As an instructor, I don't know of anyone who continued to be a diver and didn't commit to diving and bought their own equipment. Those that didn't, they just fade away never to dive again. The ones that bought all or most of their dive equipment have a much higher probability of continuing to dive (until marriage or kids get in the way
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