but had the GPL not existed when it did - would the foundations still have existed that make open source of any licence as mainstream and commercially acceptable (in some cases preferable) as they are today?
* BSD-derivative Operating Systems (code available with first releases since in 1977)
* Sendmail (1983)
* Apache HTTPD (1995)
* Qmail (1997)
* MIT Kerberos (late 1980s)
* UMich LDAP (now OpenLDAP) (1995)
Modern examples are considerably more numerous, ranging from Apache Project's Java projects, to LLVM.
The places where the FSF and GPL have really succeeded are where there was usually a strong network effect (compilers and operating systems are large individual undertakings, for instance) and limited downside to the use of the GPL.
[Edit] I realize that some fringe downvoting GPL proponents take offense at the idea that the GPL isn't single-handedly responsible for mass adoption of open source, but come on guys, there's a long and rich history of commercial open source adoption outside of the GPL.