You are making an error to be thinking somewhat linerally rather than in the power of exponentials.  You are getting more than 8x because you are getting 8 7** doublings... which makes the compounding to be even more powerful to let your investment ride upon itself.

**Note: I made a mistake in my earlier rendition since the first one does not count.. it is the base starting off point.   So we start to count doublings from the second one.  

So, let's look at the historical numbers and the timeline from 2015 to present again.

1) 0) $250  (2015)                                    1X
2)  1) $500  (2015-2016)                           2X
3) 2) $1,000    (2016-2017)        2X * 2 = 4X
4)  3) $2,000  (2017)                  4X * 2 = 8X
5)  4) $4,000  (2017-2020)          8X * 2 = 16X
6)  5) $8,000   (2017-2020)        16X * 2 = 32X
7)  6) $16,000  (2017-2022)       32X * 2 = 64X
8 )  7) $32,000  (2021-2023?)      64X * 2 = 128X
9)  8 ) $64,000  (2021-?)             128X * 2 = 256X
10)  9) $128,000  (?)                    256X * 2 = 512X

You can likely see that if you are shaving off profits at the earlier stages, then you are going to eat into the compounding (and/or exponential) component in regards to how your value would have had grown through that period of time.

So in this particular factual example the guys who bought in 2015 and had a base of $250 per BTC and who did not sell any of their BTC, they would have had experienced 8 doublings that would have brought their holdings up to 256x for a short period of time during the period that BTC was priced at more than $64k, and so then their amount of value would have come back down to 6 doublings when the BTC price dropped back down to around $16k (which would have been around 64x) and then now they are currently in the supra 7 doublings that would have been 128x when the BTC prices were at $32k, and they will be back to 256x once (or if) the BTC price gets back to supra $64k, and then if the BTC  price goes above $128k, then they will get into the supra 512x territory..

So each doubling now has much much greater magnifying effects as compared to the kind of smaller magnifying effects that would have had been felt in the first few of doublings.. so the power of the doublings tend to come later down the line, so long as the asset continues to go up in value and to have a kind of persistent effect.. .. something like a Lindy effect that suggest that the longer that something non-perishable (like an idea or a technology) is in existence and maintaining itself the more likely it is going to continue to be in existence.  

Of course, the bitcoin maximalist argument would assert that the Lindy effect applies to bitcoin more than it does to various shitcoins, but the theoretical idea of the Lindy effect is not completely absent from various shitcoins, even if some of the ideas and/or innovations of shitcoins (if they come up with any that involve anything worthwhile besides scamming people) may well have decently good chances to get absorbed into bitcoin.