Where is it started that falling uses your movement?
Also, just because you exhaust your movement for one of your speeds does not mean you can no longer move. Look at the example provided in the PHB where it discusses using multiple speeds during your movement: https://www.dndbeyond.com/sources/phb/combat#UsingDifferentSpeeds
Where is it started that falling uses your movement?
Pretty sure that's unsupported; falling is a free action, it's the landing prone if you take damage that costs you movement (on top of the cost to hit-points), at least if you plan on getting back up afterwards. It's not something you usually want to do often, as even for a Monk or a mage with feather fall it's costing a reaction to avoid it.
That said, I'm not a fan of the "using all of one speed prevent you from using others" method of resolving speeds, even though it's RAW; it's simplistic rather than actually simple. I prefer to just think of everything in terms of a base walking speed, then everything else being a cost – 60 foot flying might give two feet per foot of walking speed spent (double speed), climbing and difficult terrain gives 1 for every 2 (half speed) etc., so you're always spending whatever your base speed is because it's just easier that way.
Of course there are some edge cases that doesn't work with thanks to weirdly small bonuses and granular changes, e.g- where one speed is 40 feet and another is 30 feet (though I guess you could do 4 to 3 or such) but movement is an area where I'm generally very willing to ignore a 5-10 foot shortfall if it makes things quicker, especially since ranged is generally too safe in 5th edition, so the last thing you want to do is make melee too difficult when it's so easily at a one or two turn disadvantage already.
Lets say you have a swim speed of 20, a walk of 30, and a fly of 50.
You fly 10 ft forward, 5 ft above the ocean, to attack a flying fish, then hit him. He has some kind of reaction attack that knocks you down, falling 5 ft down into the ocean, for a total of 15 ft of flying. You swim 5 ft to the beach. This has taken up your movement and attack. Your total movement is now 20 ft, and you used some swimming last, so you have now used up all your movement for ANY type.
...
I don't get this part of the example...
- You fly 10+5 feet. You now have 35 feet of flying, 5 feet of swimming and 15 feet of walking. - Then, you swim 5 feet. You can't swim anymore, but you could walk 10 feet or fly 30 feet.
You could even use your remaining movement (walk 10, fly 30) for swimming at extra cost.
To me, the rules regarding movement aren't so much about how far you go, but rather about your speed within a six-second interval.
Is it a simplified system? Yes. Is it accurate? No. But it's fast and easy to apply during gameplay, saving us from doing math at the table.
Where is it started that falling uses your movement?
Also, just because you exhaust your movement for one of your speeds does not mean you can no longer move. Look at the example provided in the PHB where it discusses using multiple speeds during your movement: https://www.dndbeyond.com/sources/phb/combat#UsingDifferentSpeeds
Pretty sure that's unsupported; falling is a free action, it's the landing prone if you take damage that costs you movement (on top of the cost to hit-points), at least if you plan on getting back up afterwards. It's not something you usually want to do often, as even for a Monk or a mage with feather fall it's costing a reaction to avoid it.
That said, I'm not a fan of the "using all of one speed prevent you from using others" method of resolving speeds, even though it's RAW; it's simplistic rather than actually simple. I prefer to just think of everything in terms of a base walking speed, then everything else being a cost – 60 foot flying might give two feet per foot of walking speed spent (double speed), climbing and difficult terrain gives 1 for every 2 (half speed) etc., so you're always spending whatever your base speed is because it's just easier that way.
Of course there are some edge cases that doesn't work with thanks to weirdly small bonuses and granular changes, e.g- where one speed is 40 feet and another is 30 feet (though I guess you could do 4 to 3 or such) but movement is an area where I'm generally very willing to ignore a 5-10 foot shortfall if it makes things quicker, especially since ranged is generally too safe in 5th edition, so the last thing you want to do is make melee too difficult when it's so easily at a one or two turn disadvantage already.
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I don't get this part of the example...
- You fly 10+5 feet. You now have 35 feet of flying, 5 feet of swimming and 15 feet of walking.
- Then, you swim 5 feet. You can't swim anymore, but you could walk 10 feet or fly 30 feet.
You could even use your remaining movement (walk 10, fly 30) for swimming at extra cost.
To me, the rules regarding movement aren't so much about how far you go, but rather about your speed within a six-second interval.
Is it a simplified system? Yes. Is it accurate? No. But it's fast and easy to apply during gameplay, saving us from doing math at the table.