The source licensing in the early days (notably before USG) was almost certainly a large part of the key, but it certainly wasn't "given away". The flexibilit

The source licensing in the early days (notably before USG) was almost certainly a large part of the key, but it certainly wasn't "given away". The flexibility allowed for the BSD, which dramatically increased the number of hardware platforms which could run UNIX, at least in settings where an underlying source licence was available. Mt Xinu (commercially supported 4BSD for anyone with a usable source licence) was wildly popular at university labs where TCP/IP was immediately useful for local internetworking across disparate minicomputers, and the distribution included translation, bridging, tunnelling and so forth of other protocols (DECNET, AppleShare) over IP. Additionally, their port to x86 let humble 386SX microcomputers do the work of much more expensive minicomputers, and was "legit" in a way that USG argued Bill Joy's 386BSD was not. Notably UNIX distributions where source licencing was exceptionally expensive (or otherwise rare) did quite badly in settings where BSD -- supported or otherwise -- was usable. Crucially, the University of California decided to support the release of 4.4 BSD-Lite, which had all the AT&T/USL proprietary code removed, both for research purposes and in the obvious knowledge that "someone" would make Lite usable by filling in the missing functionality. "Someone" was largely BSDI, which survived USL's lawsuit; other independent Lite completions also benefitted from this victory, and FreeBSD/NetBSD/OpenBSD are descendents of one of those. Additionally, Linux came along. So I think you got it mostly right in the first line. C was mostly along for the ride; a usable compiler, toolchain, and programming environment (ed/ex/vi, lint and so forth) was included in the source distributions, and the "p" in pcc was its claim to be readily portable to arbitrary architectures (which was almost true, in part because it was written in C itself). Had it been a language other than C, that probably would have become the systems programming language instead. (Indeed, UNIX was a bit odd in that the number of general languages considered standard was tiny; for example, compare the number of languages one could use on TOPS-10 (six standard) and TOPS-20, which were never intended to run on arbitary hardware).