Imagine a world of 100 people and 100 dollars.
If the population doubles, and the 100 dollars remains fixed, the value of the dollar has increased.
If more dollars are not printed to maintain some inflation, people will slow spending hoping for 300 people the next year.
Now, in the case of BTC, it’s not global population that matters, but the population of interested users, which could go from 10,000 to 100,000,000 in months. If BTC can’t be mined fast enough to maintain a balance of coins/users as interest skyrockets, it is, by definition, deflationary.
I have heard the argument that deflation is just an unthinkable disaster for my entire life but I'm not convinced, rather it seems like a convenient excuse for countries to print their way out of budgetary problems.
That's the key. With a deflationary currency you want to keep the money, but need to buy goods. With an inflationary currency you want to buy the goods, so you only keep as much money as you need. This means that an inflationary market has shorter dependency chains, less disruptions (because value is more easily predictable) and generally runs more efficiently. Deflationary value incentivizes consuming less; inflationary value incentivizes consuming more. This is why economies grow and produce more value.
Of course this can be overdone by having an inflation that's so fast that deep production chains become impossible to manage, so you generally want inflation that's low enough that investment-based value gain is not affected, but high enough that money is not a viable investment vehicle.
Remember that money is what you have instead of something useful.
edit: For instance, if you have hyperinflation, you cannot buy a fabrication machine for your factory, because the monetary value of the machine grows faster than your ability to save money. The best use of money is always consumptively spending it immediately. This is also inefficient. So you want to balance between those two problems.
This might be true for food, but a lot of our economy is based on investments and things we don't strictly need. Why would I give money to a startup if I can simply leave it on my bank account and have a guaranteed return?
> Conversely in in an inflating currency scenario yes I might spend the money sooner than I would otherwise, but then in the future I won't be able to spend it again, why is it so much better that I buy today instead of tomorrow?
Because it keeps the money in circulation. You might spend the money, but that money then gets to a business. This business also doesn't want money lying around, so they might, for example, build something. Which gives the money to a building company. Which then gives the money back to the worker. That's a lot of movement and value creation. Also, you really want to spend or invest the money sooner, since you won't get as much for your money if you buy something tomorrow instead. With deflation, this whole reasoning is reversed - why would you spend any more money than necessary, if you can buy much more with it tomorrow? You wouldn't and this kills the economy.