At some point shit is going to simply stop shooting up.
I know you can buy bitcoins in divisible amounts. But 90% of the public has no idea they can do this, and Im not even certain that someone buying 0.00004 BTC at $10,000 is going to push the price up at all.
So my "Speculation" ? That we're reaching a point at which human beings can no longer buy in to Bitcoin enough to keep pushing the price up so quickly.
So what you're saying is, growth of Bitcoin will eventually be stopped by the limited amount of fiat people have available (or said differently, by the size of the economy.)
This is absolutely true in the long run and will happen eventually. But the way to look at it is in terms of market cap. The total market cap of BTC right now, if all coins were mined, is 21m * $1150 ~= $25 billion. That's actually a TINY amount in the context of world trade or a major currency. The global forex market alone has a daily turnover measured in TRILLIONS.
So if bitcoin ever makes it anywhere near "mainstream" usage, we still have some ways to go.