Well they could sell high strength Sodium Hypochlorite in 5 gallon drums. Maybe an SWG cleaning service. Test kit refills. Liquidators and other such dosing devices. There's plenty to keep them in business. They could even sell pool start up kits with a calculated amount of trichlor pucks and dichlor shock to bring the CYA up to strength quickly.
I predict the invention of magic pool fluid as a safer alternative to water!
Seriously, they could sell the same stuff with the fancy names for baking soda and borates because there will always be those who think that is the best and they have to have the best! A good strategy would be to sell a pool service that comes every other day to dose and maintain the pool. If they are smart they will go into the reverse osmosis business. They could make a bit off of that. Right now when they advise a drain and refill they only make money on more chems to balance the pool.
If they got on it now, they could invent a hand-held pool calculator, like a blood-sugar meter, that would measure all necessary levels and make recommendations based on your pool info. Alternatively, they could invent a sensor placed permanently in the pool, that sends the info to my PC. All tailored to individual pool specs, bacterial load and the actual products that you prefer to use.
I predict that pool stores will be much the same in twenty years. They will be full of big buckets of magic powders and expensive bottles of snake oil and staffed by ignorant minimimum-wage salesmen.
The hot product then will be some device that has expensive cartridges full of mysterious ions. But they will have sensors (hugely inaccurate) that read the levels in the water and adjust the feed rate to keep things "perfect." What will happen is that pools all over the world will end up looking chrome-plated from so many metals. And then the stores will sell polish to keep your walls shiny.