Bitcoin 101
"It's too late for me to buy in, I can't afford an entire bitcoin."
You do not have to buy an entire bitcoin. Each bitcoin is made up of 100,000,000 smaller units called Satoshis (more often referred to as sats). When (not if) bitcoin hits $1m, each sat will be worth a penny and bitcoin essentially becomes a penny stock. Sooner or later, sats will become standard nomenclature when discussing bitcoin. "Stack sats" is a common phrase in the bitcoin community, which just means accumulate as many as you possibly can because they are truly scarce. Right now, a dollar will get you about 1,600 sats.
How scarce are they?
There will only ever be ~21m bitcoin in existence, the last of which is scheduled to be mined around the year 2140. If there are 100m sats per bitcoin, and 21m bitcoin, that means there will only ever be 2,100,000,000,000,000 sats (2.1 quadrillion). This is enough for 214,000 sats per person on Earth. But, given there are millions of bitcoins that are already lost forever (estimated to be 4-5 million) due to people losing their keys to unlock them, they are even scarcer than that.
Why is it called a Satoshi?
A Satoshi is named after the creator of Bitcoin, the pseudonymous "Satoshi Nakamoto." Satoshi is assumed to be dead as none of the coins from his known addresses have ever moved, and he has been completely silent for a decade.
Conclusion
The opportunity to accumulate the hardest money the world has ever seen is still very cheap, as $150 will net you far more sats than most in the world will ever see (~245k). I would recommend this position for everyone. I truly believe you'll be extremely happy with any additional money you put in so long as you hold, but I won't recommend that for everyone. I do think it's irresponsible to not at least have a small stack of sats.