Well, it turns out that it could scale up, it just needed more power than other architectures. As long as it was only servers and desktop PCs, you only noticed it in more elaborate cooling and maybe on your power bill, and even with laptops, x86 compatibility was more important than the higher power usage for a long time. It's just when high-performance CPUs started to be put in devices with really limited power budgets that x86 started looking really bad...