How Does the Availability of Readily Rentable Hashrate Affect the Barrier to Entry for Attackers?

It drastically lowers the barrier to entry. Historically, a 51% attack required a massive, upfront capital investment in specialized mining hardware (ASICs or GPUs).

Hashrate rental markets remove this need, transforming the attack from a capital-intensive project into a simple operational expense. An attacker only needs a few thousand dollars and a basic understanding of the rental platform to launch an attack, making it accessible to a much wider range of malicious actors.