What is not a good decision for you might be a good strategy for others. For those who can be patient enough with the right prediction to know when the price is nearly peaking in the bullish market, they can utilize those opportunities by buying whatever they can afford to grab with the plan of selling off when the bull run comes. Those kinds of people happen to be short traders, and those strategies work very well for them, but most times the market usually goes against their prediction.
 
And what you considered a good decision is the time when the price of Bitcoin is in a stable state, fluctuating in the range of $30,000 Not going above that and not also going below that, to me, this is one of the toughest times for one to make an entry decision, for you never really can tell if it's going to break that stability by falling down a bit lower to the range of $28,000–$29,000 or if the price is going to break that limit and climb up a little bit. This pattern seems to be like a 50/50 kind of thing; either it goes sideways or it works in your favor.

You forgot to mention the 4th group: They are newbies who are new to bitcoin and are dying to invest in it because seeing others getting great returns. They buy at any price without doing analysis, but when we start talking about the 5th group; then it is those who adopt bitcoin as a means of payment.

There will always be someone buying bitcoin at any price and trading never stops 24/7. But even if $100k is the new ATH, there is more potential to see bitcoin price go higher than $150k.

Isn't that not too different from now?
We are also currently not required to buy bitcoin with 1 bitcoin or more because all have plans such as DCA and others although there are certainly those who can buy more than 1 bitcoin directly but such circles are a different group from us as ordinary investors.
There is nothing to argue about when asking who will buy bitcoin at $100k because even we don't really care about that and in the beginning who would buy bitcoin at $30k a decade ago and who waited for the decline like now or even when the price of bitcoin touched $15k when previously it was more than $60k.
Everything has a purpose and we don't need to bother thinking about such things because the flow will happen when the time comes.

Institutional investors are far more smarter, they are always here when the market is at it's lowest, they understood the game more than other investors, they have advisers and traders who gives them updates and good calls, these are the biggest investors in the space, do not think that they will do something stupid.

The truth is even if Bitcoin is a million dollars today some people will think that it's going to 2 million dollars soon, and they will keep buying anyway, it's easier to FOMO when price starts pumping and it's easier to shy away from the market when it's dumping.

It's better to understand the crypto market before taking an investment route yourself, there are few things many people still do wrong, and it's left for them to figure them out, if everyone knows what they are doing who are going to buy from the smarter ones?

Firstly, 100k is a certain psychological level that can instill a lot of optimism in those who want to buy bitcoin, and secondly, I’m almost sure that after bitcoin reaches the price of 100k, we will see how everyone will talk about bitcoin from every corner, and will prophesy it is unrealistic high prices, this will encourage everyone who has not yet done so to buy bitcoin with the hope, that he will have time to buy, until the price reaches a million.

Nothing can be said for sure but expect the price of Bitcoin to move up to your given price. If you really want to invest then I would say Bitcoin is the best choice. if you want to buy other coins for short term profit then you can make more profit but in opposite direction. The risk is too high.

The same question could be asked about who buys bitcoin at 30.000USD. The answer is simply: everyone that needs bitcoin at that time. People get used to any kind of prices pretty fast and see them as normal.