It's not in your best interest to buy and never sell. Current investors are just incredibly short sighted. If bitcoin isn't usable by (and useful to) the majority of the world, it will never get enough investment to serve as a buffer against the drastic price swings caused by speculators.
Smart investors know that simply holding coins isn't enough in the long run. Finding new use cases and encouraging adoption and use is the only way crypto stands a chance.
When you start to account for other things like lost and unrecoverable coins, it becomes severely deflationary and much sooner. Things like grandpa taking his encrypted wallet to the grave or in the case of bitcoin BTC, small coins that can't be moved because the fee is higher than the coin's value.
At the time of writing this, coins worth less than ~ $6.15 (USD) aren't able to be spent*. That means if you bought a $4 coffee with BTC and paid with a tenner (a coin [input] worth $10) , you'd lose the change. It would remain unspendable until the network starts unclogging.
* By "arent able to be spent" I mean, reasonable fee estimations for getting the transaction into a block within an amount of time that you can be sure the transaction won't get dropped from the mempool. I used https://bitcoiner.live to estimate the fee which was the most generous of the three I looked at.
Any currency with a limited supply will tend to be deflationary over time, though not necessarily experiencing a deflationary spiral depending on the overall economy around it. An active economy will see enough movement of the currency that deflation will, hopefully, be small. But an inactive economy (like bitcoin's) with people hoarding rather than spending will see higher-than-healthy (from an economic perspective, it's great for the hoarders if they can use it later) deflation rates.
Now, you can make the argument that that’s not really inflation - that BTC won’t predictably ~double in value every year going forward, and therefore one shouldn’t be afraid to spend it - but the idea that the tiny rate of increase in supply should be a major factor in that decision is pretty silly. Also, it’s not obvious that it’s being mined faster than it’s being eliminated due to lost keys.
But bitcoin is more of an asset than a currency, so speaking of it in terms of inflation / deflation leads to oddities, like this.
Assets inflate and deflate. It’s not an oddity.
Bitcoin adoption is a deflationary process because an increasing number of people are competing over a fixed (ultimately) supply of Bitcoin.
If more people want Bitcoin, the price goes up. Why do more people want Bitcoin? Because the price goes up. The cycle reverses when the price starts going down.
Define best interest here. I mean saving money and investing it to make more money is also in everyone's best interest, but that is certainly not the case. So why is that?
Holding BTC is the same as the last one. The holdings are not invested in anything remotely productive, it's just speculation on the future value.
Bitcoin’s deflationary nature discourages spending and selling, by definition. The high transaction costs discourage it even further.