This is slippery slope fallacy. The argument against mandatory donations is exactly that - it's makes donations mandatory, while opt-out gives you choice and just changes the default.

If you seriously fear for your life as a registered donor, it should be not too much work to check one box.

If there is mandatory donation, or even if there is opt-out, wouldn't that increase the supply to meet the demand, thereby reducing any incentive for profit on the sale of organs and eliminating any motive for corruption?

If the end justifies the means, then I think it's irrelevant whether the change causes demand to be met so long as more people get the organs they need. On the other hand, if you don't believe that the end justifies the means, then it's completely irrelevant because you have to respect people's choices over their bodies (even posthumously) even if you think they are misguided or 'selfish'.

Only if the supply actually increased enough to actually meet demand. It is far from clear that it would.

Why does there have to be a next step?

Both opt-in and opt-out offer you the same degree of choice; neither is a step towards mandatory organ donation.