After a good warm-up, I can perform a high mawashi geri pretty easily. Without a warm-up, however, I can barely lift my leg higher than belt level. In a real-life situation, you won't have any time to stretch your ligaments (notwithstanding clothing limitations related to your clothes, etc.). So what's the point in practising high kicks if you can't use them in real life for self defense?
"what's the point in high kicks if you can't use them in real life"
Just because you can't - currently - doesn't mean others can't, or that you couldn't.
It can vary a lot by kick - for example, some people (me for example) find mae geri (front kick) much easier to deliver high and "cold" than mawashi geri, or even uchi and/or soto geri, or ushiro mawashi geri. Others are just fine with mawashi geri.
More generally, there are lots of moves in karate that you can't use properly if you don't put the time in maintaining your preparedness - e.g. you can't punch harder targets full power if you haven't conditioned your knuckles. Should we stop punching?
For high kicks in particular, you may be able to find a routine that loosens you up throughout the day, such as some static then dynamic stretching when you hop out of bed in the mornings, but not saying what works for some others will be exactly what you need.
Whether you can stay limber throughout the day may also depend on lots of factors. If you're a desk worker it's less likely that if you're say a gardener, dancer, delivery person - where you can probably either find ways to integrate some stretching into your work, or do some in what you're working between bits of work without drawing too much attention.
I don't have this problem. Sometimes my legs get sore, but that goes away after a couple of days. If you stretch every day, and even do maybe ten high kicks with each leg in the morning, and if you can, after lunch, that should be enough to use them whenever you want.
Remember, self defense isn't the only purpose for martial arts. There are several techniques in many different arts that might score points in competition but wouldn't help you in real life. It's your job to decide whether or not something would be useful in a self defense situation. If you think that your high kicks wouldn't be useful for self defense, then resort to something else that you are more comfortable doing at any time.
I do still believe, however, that stretching daily will eventually lead to being able to high kick whenever you want. Here are some stretches that will help you get higher kicks:
- Butterfly stretch(sit with your legs in the shape of a diamond, then bring your chest to the floor.)
- Touch your toes while standing up(without bending your knee.)
- Touch your toes while sitting down and your legs together.
- Spread your legs apart while sitting down and lean to the middle, then each side.
These should be done for about 10-20 seconds each, with small movements each second to prevent cramps.
All in all, if you keep up a daily stretching routine, you should be able to use high kicks at any time you like.
Assuming you don't need convincing that high kicks are useful (and watching an MMA bout does show that they can be fight-enders, although they also can be very risky outside of the ring and its rules), if you can kick after stretching, the odds are you can kick in a crisis, although you may regret it the next morning. Adrenaline is a hell of a drug, and half of martial arts training is removing mental blocks by virtue of performing a movement slowly in a controlled scenario to convince your body you can handle it. Most people can physically kick over their head. But, without training, you have more difficulty slowing it before your muscles and tendons strain, so your brain clamps down on the movement. In a crisis, and/or with training, those blocks go away.
So what's the point in high kicks
They are sport only, and if you can do a high kick well, you can do a low one well. Don't use high kicks outside a sporting environment, poor surfaces will eliminate your grip and you'll end up on the ground.
Pre ~1950s nearly all kicks were low. Most around knee level, and primarily for manipulating the opponent's body than trying to cause impact.
If you're looking at kata, imagine the same kick at knee level.