As an author of some niche open source, I've tried simple licensing like that (e.g., "GPL, or ask me about other license"), but the software had only a few commercial users that I know of, and I never made a penny that way.
You need commercial users who want to use your stuff enough, and the GPL/LGPL/whatever license won't work for them, such that it will get pushed up to someone who'll follow through and negotiate a license.
(You also need it to not quietly be used by parties ignoring the license, like has happened many times with Linux and certain GNU-ish libraries that would be expensive to reimplement.)
I've advised a number of clients away from public-private licensing toward simply approaching their clientele directly and pitching traditional software deals. Some who manage to sell licenses that way choose to release their work under copyleft or restricted terms, after the fact.
The tell is a known, small, approachable target market. Especially when the developer already has positive relationships with important would-be customers.
You need commercial users who want to use your stuff enough, and the GPL/LGPL/whatever license won't work for them, such that it will get pushed up to someone who'll follow through and negotiate a license.
(You also need it to not quietly be used by parties ignoring the license, like has happened many times with Linux and certain GNU-ish libraries that would be expensive to reimplement.)