OP, back in the day there were people just like you who couldn't even fathom the idea of bitcoin ever breaking $100, much less $1,000. And $10,000 per bitcoin back then was considered "ridiculous". LOL Just search old threads in this section if you don't believe me.
No need to wrack your brain over it. Just buy the dips and enjoy the ride.
I think though that there is a plateauing of sorts that happens the higher up we get. There is more resistance after a while. For someone to get a BTC for $1 is easier than for someone to get it at $10k or $15k for example. So yes, while it may have been unfathomable for it to go to 100 or 1000 for example, the price growing 10x from 100 to 1000 was easier than the price growing 10x from 10k to 100k. There is far more resistance at this level.
I think ultimately the price will grow. We might hit 50k in 2018, but we probably will not see the 1800 - 1900% increase that we saw in 2017. No reason to be concerned though. Unless you have hundreds of BTC in your wallet, the % increase isn't what matters. Think of your profits in regards to what you can buy with them, not your ROI. Even a $1,000 profit is very significant because it can buy a good amount of stuff.