The answer to this question is complicated. Monopolies can be protected by startup costs, but in most cases this is not the primary way by which a monopoly is protected.
Startup costs, like any other barrier to entry, can act as a protection for a monopoly. However, unlike other barriers to entry, such as exclusive access to resources, natural monopoly economies of scale, and government regulation, startup costs are not always enough to guarantee monopoly profits.
Startup costs include any expenses related to launching and maintaining a business—like initial capital, salaries, facilities investment, research and development, administrative and accounting processes, legal fees, and more. These costs can represent a significant barrier to entry, particularly for smaller firms and startups that may not have access to the same level of investors to finance these costs. This can then become an effective tool for large firms to maintain their market position, as smaller rivals cannot afford the same upfront costs.
An example of this can be seen in the telecommunications industry. The high costs associated with deploying infrastructure (i.e., laying cables, setting up poles, renting out public spaces) often allows large firms to monopolize markets, as rivals may not be able to afford entering the market and competing at the same level.
Ultimately, while startup costs can provide some degree of protection for a monopoly, they are unlikely to be the only factor protecting a monopoly. Other factors, including but not limited to government policy, legal barriers to entry, and exclusive access to resources, may all be taken into account when deciding if a monopoly is protected by startup costs.