I don't see any mention of "modular" or "boundaries" in the post you linked, so I'm assuming that it doesn't add extra context to your point.

You say "very limited tools for enforcing modularity boundaries", which I'm going to assume means that you believe that Lisps have constructs for creating modularity boundaries (e.g. unexported symbols in Common Lisp), and just don't enforce them (e.g. I can still use an unexported symbol in CL by writing foo::bar), in which case - I don't think that this is actually an issue.

Programmers are capable of doing all kinds of bad things with their code, and shooting themselves in the foot, yet I've never seen an indication that the ability to shoot yourself in the foot with a language noticeably contributes to its popularity (see: C, C++, Perl, Ruby).

Moreover, specializing to Common Lisp, it's not like CL allows you to accidentally access an unexported symbol in a package - you have to either deliberately use :: (which is bad style, and takes more effort than typing :) or you get a hard crash. This is opposed to the above listed languages, which allow you to shoot yourself in the foot in a large number of extremely diverse and interesting manners, often without giving you advance warning - and yet are still far more successful.

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I don't believe that the lack of success of Lisps are due to technical deficiencies.