> For example, if the customer runs a legal firm, they should buy a CRM with functionality targeted towards legal firms

Even those won't be specialized enough, a legal firm that handles corporate megers and acquisitions has very different processes from one that does class action litigation or criminal defense. Successful CRMs are not opinionated, but are customizable - requiring an army of consultants to deploy and an internal role to support. Small companies may be coerced to using opinionated processes (quickbooks + whatever process the part-time bookkeeper wants), but every medium-sized enterprise or larger is a snowflake

Again, not an area I know much about, but: there are going to be hundreds, if not thousands, of medium sized legal firms in the US handling, say, criminal defence.

They can't be that different from one another that they would require bespoke software, other than an off-the-shelf solution for criminal defence lawyers? Or is there sufficient difference e.g. state to state that this would never work?

While selling the simpler product, the financial position of a company selling "Criminal Defense CRM" is substantially different to a more general "Legal Firm CRM" one; both would be competing with "Salesforce for Law Firms[1]", with the first having the smallest Total Addressable Market.

1. https://www.salesforce.com/uk/solutions/industries/business-...