The speculation is on that the alignment of central banks globally is towards savings debasement and non-custodial property. HODL is a rallying call to exit the current monetary infrastructure into the next one, one that specifically allows for a free market for money. Bitcoin is a bet on that transacting with bitcoins will ultimately be cheaper and with a vastly better security model than all existing infrastructure, from the concerns of a nation state down to microtransactions.

They're not speculating that it will go up for no reason. The investment thesis is that it solves real world problems, and that everyone who thinks it doesn't sounds like someone comparing fax machines to the internet in the 90s.