Could create it and make a profit selling it.
If the cure generates less profit than current treatment, and if you’re currently selling treatment, then that’s not going to be worth it for you.
If you don’t, well you still need to get it to market, which requires substantial investment.
My point is that the described scenario is not entirely unlikely. For example, type 1 diabetics in the US need something like $600 worth of insulin, every single month for the rest of their lives. Say that’s 60 years, you’re looking at $430k, not even counting needles, sensors, and other stuff they might need (or absolutely need). Average patients won’t be able to cough up nearly as much for a cure.
Pharma has just been way faster than tech in realizing that the subscription model has far better yields.
Your idea only works if you believe there is a global conspiracy of businesses and scientists who all agree to embargo research into cures, and who are all willing to forgo any profits on the cure while also recognizing any scientist and company not in their conspiracy could wipe out their profits at any time.
There is no "global conspiracy" required at all. Not actively doing research towards reducing your own profit is enough.
Outsiders could theoretically come in, yes. But the hurdles are huge for biotech startups, and of course the established players are lobbying to keep it this way, so I won't be holding my breath. It's different from the tech world.