^Buying Bitcoin with lump sum is the best strategy but the problem is not everyone can afford to dump at once which is why DCAing is very effective for accumulating Bitcoin even for the retailers. And also I wouldn't say selling at high is wrong because that's the basic of making profits by selling an asset while we can't really draw the line which is high and which is low due to the highly volatile nature so all we can do is just go with the instinct and try to make best out of it.
You can only describe something as best if that particular thing is convenient for everyone and has more advantage than the disadvantage, so saying lump sum is the best will be wrong because it is not convenient for everyone and everyone can not afford it and so the DCA still remain the best because everyone can use it to accumulate Bitcoin whether small or big and whether rich or poor. However, selling at high is not actually wrong that if you have hold for years and your investment is very matured but it is not right for low coiner to sell the little coin they have instead of thinking on how to accumulate more and any low coiner who sells when the price is high is obvious the person is a trader not an investor.
You have made some valid points, but I like to add some some few words. Whether lump sum or DCA is best it really depends on individual circumstances. Lump sum investing can be advantageous for those who can afford it and are willing to take the risk, as it allows for full market exposure right away. However, DCA is undoubtedly more accessible and flexible, especially for those who want to minimize risk and steadily build their portfolio over time.
As for selling at a high, I agree that the Long term holders with substantial investments may sell to realize profits or rebalance their portfolio, which can be a strategic move. On the other hand, for those with smaller holdings, the focus should ideally be on accumulating more Bitcoin rather than selling prematurely.