Why sell something that is gaining purchasing power
Because if you sell it, and buy some products or services, then you will benefit from it. But if you never sell, then you may die as a millionaire. Which means, that the only people, who will enjoy it, would be the ones who will inherit it later. Which is usually bad, especially if they put no effort into getting it, because many of them can lose it, just like many lottery winners did.
Money alone is not the goal, it is the way to reach some goals. If you focus only on having more money, without ever thinking, what do you want to buy, then you will never enjoy it.
Do you truly understand why Bitcoiners say, “Never sell your Bitcoin”?
No. If you replace Bitcoin with anything valuable, then it still wouldn't make much sense. Also note, that the market would be totally different, if everyone would never sell. To make a transaction, you always need a buyer and a seller. If you want to buy Bitcoin, then you need to find someone, willing to sell it.
you should never sell Bitcoin because its exchange rate would keep rising
It was historically true, but it doesn't have to always be the case. It was rising, because a lot of people put their time and effort, to make it happen. But when everyone would focus on taking profits, without thinking about improving the system, then the competitors would win. There are many dead altcoins, because their creators made a lot of bad decisions. However, as some people will start learning from past mistakes, new ideas will put more pressure on Bitcoin.
So, the exchange rate would keep rising, only as long as people would keep improving Bitcoin. Otherwise, it will hit some limits, and people will switch to something else. If all people in the world would start using on-chain Bitcoin today, here and now, the network would be stuck, and would process only whale transactions with the highest fees. Which means, that adoption is not everything, and some technical improvements are needed, otherwise the system will start collapsing, or centralizing.
And when it comes to centralization, it will bring us back to when we started. Big exchanges, and other custodians, will have a huge power over their users. Which means they could behave like banks, and process coins they don't have. And then, when they start collapsing, their users will become poor. Without scaling, some of them will be always screwed, which will also affect the price. And then, it wouldn't matter that much, how many dollars people would need to pay per coin, if they would have a choice between expensive first layer, or custodial second layers, and if daily usage would give them banking experience, more than P2P experience.
So, if you wonder, why the price was climbing so high, and then suddenly dropped, then this is also part of the answer: because there are some scaling limits, and when Bitcoin hits them, then it can collapse, or centralize. If developers will solve some of these problems, then the price will go up. But if they don't, then it will go down. Or it will scale through altcoins, which is also why Bitcoin Dominance charts can sometimes show you values below 50%.