Frankly, from a product perspective it's short-sighted. Let people leave. Find out why. Fix the problem(s). But if you hold them hostage you're holding your own product back _and_ you're creating ill-will towards your brand. Lose, lose.
That's not forgivable.
Yes, there are higher migration costs if you use Squarespace. For a lot of people those are reasonable in exchange for lower up-front and maintenance costs. People should absolutely consider the lock-in factor but there's not a universal correct answer.
There's always a tradeoff.
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.