It's not a huge change though. .dev was a new, never-launched TLD, so there were no existing real domain names to break with the addition of HSTS preloading. Established best practice for decades at that point was already to always use real domain names (or subdomains thereof) or specifically reserved test domains/TLDs (see RFC 2606, published in 1999) for testing/development/local networking purposes.

So yes, we didn't anticipate how many people weren't following the best practices, but that would have been hard to determine prior to doing the thing anyway. There were also lots of people who had the mindset of "We won't change anything until it stops working", so in some sense a lot of it was unavoidable. See e.g.: https://github.com/laravel/valet/issues/204 https://github.com/laravel/valet/issues/294 https://github.com/laravel/valet/issues/431 (note that Laravel users were responsible for a non-trivial fraction of the total problems experienced, and that we only discovered all this post-HSTS-preloading). The problem was repeatedly pointed out and the maintainers refused to fix it until it actually broke. So, inevitably, it broke, and then they fixed it.