That is the reason why anyone can't just create a new coin and expect everyone else to value it, but that doesn't mean there can't be any viable competitors. And if a competitor with a sufficient advantage got popular enough, it could take the lead.
For example, the hack Bitcoin used to become popular was to be deflationary, which encourages speculation. That got the value up before it was cool, but it stays deflationary -- even gets more so -- once it's popular, and the consequent speculation causes high volatility. Once you're popular, being deflationary becomes a liability instead of an asset.
It's possible for a coin not to be deflationary and have the supply set by computing costs. If the value of the coin falls below the energy cost of creating them, people stop creating more of them and the value stabilizes there (so it doesn't keep going down). But as demand for the coin increases the value rises above the threshold and people start mining again (so it doesn't keep going up).
That stability would be a huge advantage. But it's not possible to convert Bitcoin to that because its deflationary nature is priced into its value. Making it non-deflationary would make it worthless to speculators, which would eliminate most of its current valuation.
So all it would take is for a non-deflationary coin to become popular enough for people to trust it not to disappear overnight and the people using it for transactions rather than speculation would prefer it over Bitcoin.
