The question is, can Apple do something in the form of an iOS update to correct this in the MagSafe puck or the 20W charger?
If, for example, something is making my iPhone X draw only 3W, can it be told to draw 7.5 or 15?
Yes, it's possible that Apple could make changes in a future version of iOS to provide better compatibility for older devices with the MagSafe, though I wouldn't expect to see parity with the new ones in the form of 15W.
It made adjustments in 13.1 to mitigate concerns about potential interference, albeit at the cost of 7.5W charging with non-EPP chargers, which could be perceived as removing a loophole as well.
It's probably unlikely to see changes in the firmware in the MagSafe or the 20W or 18W adapters, if it was even feasible from an end user level. As far as Apple in concerned, those combinations work as intended, sold as a solution for Apple products.
Apple's priority is not to go out of its way to ensure that the MagSafe is compatible with other makers' devices. Read between the lines, and there are no explicit claims of compatibility with anything but Apple products. Others
may work, but are unsupported.
Contrast that to Samsung's chargers, or the Pixel stand, which do explicitly claim support for Qi compatible devices.
So, if anything, Apple may tweak iOS to make its iPhones work better with MagSafe, but the rest is not a concern.
As far as MagSafe's compatibility with 3rd party adapters, there are apparently some kinks to be worked out there as well. The accessory makers are the tail, not the dog, so what will probably end up happening is that they will tweak their products for MagSafe compatibility in general, but it will take some time, certainly more than the two weeks that these new products and the new ecosystem has been in existence.