Which of the barriers to entry can last indefinitely and which are more likely to eventually erode such that a new entry can take place?

What will be an ideal response?

ANSWER:

Barriers such as economies of scale and government restrictions can last indefinitely since the first depends on technology and the latter on government action. It is unlikely that a firm can maintain control of a key input indefinitely, and patents expire after 20 years. Government regulations and tariffs can last indefinitely, although a change of government could knock these down as well.