That's a good bitcoin allocation, I also try to have 70-80% of my crypto money in bitcoins. Bitcoin is still the most popular coin and offers a good return outlook with the halving next year. I expect that most people are waiting for next year before consider selling any coins. Waiting for the price to reach the ATH is another strong level after which people could start selling again. The problem is that if many people focus on such fixed levels to take profits in bitcoins is that there will be a lot of selling pressure. It could even get so bad that there are not enough buyers in the market and prices would drop sharply. Let's assume we surpass the ATH next year and directly the next day a large number of longterm investors want to sell their coins. Without many buyers in the market this could lead to a panic and collapse of the price again. I try not link my profit goals to fixed bitcoin prices as I still believe the longterm trend is upwards for Bitcoin.
Now, what about the big sales after ATH? Its possible that it will happen, and your fears are valid. But keep in mind what the market is all about: supply and demand. People who sell something today may buy it tomorrow, especially if they see value in it.
Someone who sees the bigger picture and is focused on the long game instead of the short-term drama is nice to see. Hold on tight, and let's ride the Bitcoin wave together while we learn how it rises and falls.