Speculating that prices are too high is as natural (and necessary) a part of any well functioning market as speculating that they are too low. For stocks downward speculation is called "short selling" and it has often been the boogeyman of regulators and politicians who want to score cheap political points based upon people's ignorance of how free markets work and which come at the expense of the market's function as a whole.

Consider: what would happen to stock prices if short selling were outlawed? With no countering force to long speculation, stock prices would on average be too high, and in specific cases (eg, the GME long side manipulation we saw a while back) certain stocks would become MASSIVELY overvalued.

Why is this a problem? Because one of the most important functions of any free market is to generate correct prices. Correct prices are perhaps the most important reason that free markets outperform centrally planned markets. It's not hard to see that correct pricing requires that speculators have the ability to express that prices are too high (by shorting) as surely as they can express that they are too low (by going long). Removing shorts distorts market pricing and degrades the ability of free markets to efficiently allocate capital.

Aside from market efficiency arguments, people often feel that banning short selling will somehow protect them because they reason their stocks will be 'less likely to go down' because the 'greedy speculators' out there 'manipulating markets' will be banished. Banning shorts will not prevent price manipulation; for example pump and dump scams like the GME fiasco will work just fine without shorts in the market (except that they become much worse). Reckless long speculators will continued to be fleeced, and investors will on average get significantly worse prices on their investments.