HODLing is using bitcoin. It is probably the primary motivating use-case for it. To HODL is not necessarily to speculate, but simply to save money. Money is multi-purpose, and saving is a valid use case as much as spending is - although modern monetary theorists think that saving is not a valid use of money, and thus have fixed their money systems to making saving impossible.

There is a distinction between HODLing and merely speculating, although there is overlap in these uses. A speculator is typically somebody who wishes to see their USD holdings increase in the short to mid term as a result of exchanging bitcoin on the market at the right time. A HODLer is somebody who expects their 1BTC to still be 1BTC in 1 year, 4 years, 40 years, ...

There is no implication that there needs to be a financial collapse. There is only the implication that fiat money is guaranteed to be devalued through inflation, which is decided upon by self-interested, unelected, unaccountable men in the shadows, who benefit from being the issuers of new money at the expense of the later recipients of the new money (The Cantillon Effect).

If $1 now is worth more than $1 in 1 year, or 4 years, or 40 years, then anybody of sound mind is not going to save in dollars because their purchasing power when they come to spend the money will not be worth the effort they underwent to earn it. One option for people wishing to save over a long period is to invest in risky enterprises - which is speculation, as much as any investment in bitcoin is. Those other markets are also subject to manipulation, insider-trading and other ill-doings which don't benefit regular savers.

But bitcoin presents an option which is intrinsically different from all of the others: it has an absolute maximum supply which is directly measurable by anybody, which means that its value is subject only to the subjective opinions of market participants trading bitcoin for other commodities and any shadow bankers are absolutely powerless to change this.

The BTC/USD exchange rate is not what is interesting. There are potentially infinite dollars, but there are potentially only a maximum of ~21M bitcoin and no more, ever (but possibly less). There has never been such a hard form of money in history, and even the closest analogue, gold, has been subject to inflation on the discovery of new gold mines, or even through manipulation of the matter (fools gold, coin clipping, etc). Unlike gold, Bitcoin is also easy and cheap to verify for anybody.

It gets more difficult to release new bitcoin with time due to this block subsidy decrease. There is a race to accumulate as much as possible as early as possible under the concern that it will cost you much more later (either in money, or in labour) to obtain the same amount. You're effectively bidding for a share of the potential maximum of 21M, and those shares are getting harder to obtain. To give a historical account: if you had purchased 1 BTC in dollars 10 years ago, it would've only cost you 1/8000 the amount now. Translate that into labour, and it means you'd be working for several months to years to obtain the same share which could've been obtained for 5 minutes of work if you'd done it sooner.

There's still ample possibility that Bitcoin could see 10x, 100x or even 1000x gains over the next years/decades. If you hold fiat money in a bank account, you are risking missing out on all of that. Are you willing to take such risks? How is holding dollars no less of a risk than holding bitcoin?