I agree with the thought of low resolution being good enough, but this is a by-product of convenience. It is not the main reason for the multitude of new divers using phones or action cameras like GoPro. They are simply easier to carry and use. Smaller profiles and ease of use are driving the entry level market. There is a reason why phones, action cameras, and products like Sealife are popular. They are easier to carry and use. Action cameras now have 4K video and advanced features in a small profile. And, everyone has a phone which takes very good pictures and video. The entry level market is where the money is and why there are so many products and accessories. Also consider how often the average person goes diving, maybe once a month for two dives that day and perhaps skipping the winter, bad weather, or holidays. The average user does not want to invest in a big expensive rig which will get minimum use.
I have an older DSLR in a housing with two strobes and it is a pain to carry it around and push it through the water. Forget about swimming against a current. Newer features such as higher resolution and 4K video are not preventing me from upgrading to something newer or better. Size, convenience, and cost are the issues for me. Cost is a big issue because I don't make a dime from the pictures I take, but I would be willing to spend some money if I thought I would enjoy using it.
I like doing beach dives, but I won't drag my rig out to the reef, swim around for an hour, and then push it back across the sand to the beach. I usually do two dives since my gear is already wet. If the conditions are not good for photography, there is no way I will leave my rig in my vehicle while I do a second dive. I would rather leave the rig at home and bring something smaller which may sacrifice quality. For the phone or action cameras, grab stick and go.
DSLR or mirrorless $$$$
strobes or video lights $$$
accessories such as electric or optical cords, focus lights, etc. $$
*** OR ***
phone or action cameras $$ (everyone already has a phone)
case $ - $$
Also consider this...
### camera and housing
First, choose a lens such as general purpose, macro, or wide angle. Wet lenses make this a bit easier.
Are you shooting mid or wide angle and will balance ambient light? Do you need to include red filters to compensate for the color loss?
Set shutter speed, aperature, and ISO for proper exposure.
Set white balance at depth for best color representation.
Do you have strobes or video lights which will now blast white light which will enhance the reds? Do you need blue filters for the lights?
Focus and compose picture or video.
*** OR ***
### phone or action cameras
The camera manufactures creating the higher end products are doing so for a target audience, not the average consumer. I agree there will always be a small group of people that want to upgrade to something more than a phone or action camera. I also agree the people earning money from photography and video production will be the ones to upgrade to something better in the future. But, the vast majority of users will accept less for the convenience and lower cost. I think the camera manufacturers know this and some don't see a future for themselves.