Eth’s no max supply causing price stability?

This might be a completely n00b question, but please help: I just don't get why people think that at some point, when there is a 'mass adoption,' or whatever, that Eth will keep going up over time, as a store of value, if there is no max supply guaranteed. Can someone help me out on this one?

Can the validators, or whoever it is who decides, just decide to add billions of extra Eth into the system one day, if they wanted, and crash the value of Eth? I'm sure they wouldn't crash it, but can't they just chose to keep it stable forever so that the network can be utilized for it's use purposes, rather than a store of value like btc? Is this a reason why it just keeps fluctuating over the years between the same figures? Isn't the purpose of Eth for it to be used for contracts trading? So why spec on it when supply can be injected in at any time?

What am I missing here? I'm sure there is a reason why people are speculating on it right?