How open source licenses increase or curtail reach of the software

Relicensing from a permissive to a copyleft license curtails the potential reach of the open-source software, while relicensing from a copyleft to a permissive license increases its potential reach. In the abstract, this is easy to see: Having less requirements on the use of the software allows more uses and hence increases reach.

The confusion, as shown in the discussion around the relicensing of Immich, stems from conflating two types of users into one, and then talking past each other because the two opposing factions are talking about different things.

The two users of open-source software are:

1. End-users simply use the software for its intended purpose. The software typically is an application then (sometimes a component if you create your own application from it).
2. Distributors enhance the software with a closed complement (hardware or software or service or a combination thereof) that they pass on to third parties (their end-users).

There are no restrictions on end-users: They can just use the software. It doesn’t matter whether it comes with a permissive or copyleft license.

However, for a distributor, a copyleft license makes it impossible to sell further software as a closed complement (i.e. a vendor who enhances the open-source software with some proprietary modification or enhancement). Hence, vendors with this business model will simply not use copyleft-licensed software, cutting off its further distribution and reach. It is that simple.

People sometimes argue that a copyleft license will appeal to more developers and hence will increase its actual reach, even though its potential reach is lower than that of a permissive license. This argument hinges on the assumption that developers prefer copyleft licenses over permissive licenses, which is not true, as the rapid growth of the use of permissive licenses over copyleft licenses shows (local screenshot copy). (Compare this with 2014 data when the GPL was still dominant.)

If you are an open source programmer and want your software to go far, use a permissive license. If you are a vendor who wants to curtail specific competing uses, or a private person who wants to prevent the described specific commercial use, use a copyleft license.