There is no ideal or correct time for investment. Every time is appropriate if you want to invest long-term. Whoever bought at 2017 ATH can sell now and his investment will have been lost, but we can say that this time is the best time to achieve the maximum possible profit in the shortest possible time?
In theory, it is correct, but the price may be in the area between 25k to 32k at some point in the beginning of next year before the upward line continues. Therefore, now is an appropriate time to achieve maximum profit, but it is not the best.
Calculating Bitcoin's past record gives some idea of when the price of Bitcoin is likely to rise. Usually we see that every 3-4 years Bitcoin has a high pump and ATH build up. But it's hard to say when that will happen. So one should always plan for long term investment and only long term investment can bring a good profit. And a person who only looks for a right time to invest will never find that right time. A few months ago when the price of bitcoin dropped to $15k we were still wondering when is the right time to invest in bitcoin and the price of bitcoin will probably drop further but from there it has now rebounded to 34k which is 2x+ from that time. So rather than finding the right time to invest in Bitcoin, it is wiser to invest instantly and hold patiently.
In the past, we've seen which seasons saw the biggest gains in Bitcoin prices and when the market bottomed out again. If we analyze those past markets then we will surely understand that you are right that after 4 years a new ATH of the market is formed. But it is difficult to say when that time will be because no one can say it, but if we have patience then surely it will be possible to reach that stage. It is true that long term investment always helps in getting profit, but short term investment never able to give huge profit. A long time ago the price of Bitcoin reached $15000, but then we saw that people were very scared to invest even then, and they questioned whether now is the time to invest in Bitcoin. But if those successful investors had invested at that time, they could have earned a good amount of profit by now, just like today, since the price of Bitcoin is $35000 dollars, even if you invest now, you can earn a good profit later. It would be best to invest in the current market with a long-term plan rather than looking for time to invest in Bitcoin.