If bitcoin has only gotten more expensive over time, it's probably going to be problematic to buy a whole bitcoin in the future. The only thing that can be done is to follow cycles and sell when the top is up, then buy at the bottom in parts. But even in that case you would need to spend a lot of time.
Buying a coin at a relatively low price and selling it at a higher price is what we generally consider trading. By investing we mean holding for a long period of time buying a reliable coin. There is a substantial difference between trading and investing.
Trading is usually conducted by people like a business as in business a commodity is bought at a low price and sold at a high price, similarly in the case of trading any coin that is bought at a low price is sold at a high price. If investors are asked questions about investing, they will never say that they bought a particular coin and sold those coins at a higher price after a few days. By investing in Bitcoin we mean holding it for the long term as we mean Bitcoin. If the investment cannot be sustained for a long time then I think the investment is incomplete.
You may have guessed that at the current rate of Bitcoin price it is impossible to buy all Bitcoins at one time. Nothing is impossible, everything is possible for humans. There are many people who buy many bitcoins and hold them for a long time. Our investment holdings are negligible compared to the way large investors hold their investments for long periods of time. Those with enough money to invest can purchase multiple bitcoins if they wish. If those who don't have enough money invest regularly, then one day they will own one BTC, only investors need to be consistent.