The people that have made the most money are those that bought and held for a LOOOONG time.
And continue to hold.
When Bitcoin hit $1k, even I sold, but it was still 5% or less of my total holdings.
Was that a failure? I could have sold 100% and bought back in now for less than half the value!
I'm not sure the price is necessarily going up, but I would never spend more than 20% of my net worth in fiat to buy Bitcoin at once and I would never spend more than 20% of my Bitcoin to buy fiat at once. :/
And there's the hard part. Knowing when to buy and sell. I bought a 7950 for mining (had to get a new PSU as well, so $400) when Bitcoin was $50. I could have bought 8 BTC. Instead, since that time I've mined less than 0.5 BTC, and it's already almost not profitable anymore. I essentially lost 3.5k (as of today's rate) by making the wrong choice.