So they should have an export function to export all the supporting Squarespace code? Yes, that seems like an unreasonable expectation.
If being able to easily lift and shift is important to you, no one is keeping you from hand-coding your site and/or using some sort of static site generator and hosting it on a generic platform.
If you write an application using AWS-specific services, you're not going to be able to trivially migrate that either. It's a tradeoff.
