One pretty big cost is the risk of confusion when visiting "somebrandname.sometld", especially if it's a different entity from the ".com" domain.

It can also be a cost (both moneywise and as a time sink) for brands wishing to protect their identify, if the number of tlds increases with a magnitude. (Multiplied by the number of misspellings and hyphenations etc).

It's probably likely to break a lot of (buggy) software (just try entering an @example.museum email around the web today).

Non-technical people are likely to be confused (so increased support costs, re-education, etc).

What happens to a gTLD and all the registered subdomains, when its sponsoring corporation goes under?

The DNS root zone grows, which may have undesirable costs for caching nameservers and DNS traffic.

It feels like a waste of energy and time that a non-profit like ICANN could have used for more constructive work.