A producer who strives for product innovation for quality competition, as opposed to price competition - is branded as an imperfect competitor. Many economists may not like quality competition, but consumers do. Take King Gillette and his revolutionary safety razor of a half century ago, for instance. The individual consumer can better decide such questions, for only the consumer knows exactly what he or she wants. The requirement of free entry also does not correspond with competition in the real world.
Any entry involves cost, of course, as does all economic activity.
But to posit a model of perfect competition in which the costs of entry are very low, runs against common sense. Another example of a barrier to entry cited by quite a few economists is advertising. True, but they must do so in order to win the consumer's acceptance. For new entrants, advertising is frequently a vital means of gaining acceptance. Restrictions on advertising, which are recommended by some economists, would hurt new entrants and potential competitors.