The three companies you list are horrible examples. IBM is kind of a UNIX vendor, sort of, but not like Sun or DEC. They sell solutions, and the solutions that use AIX don't overlap with what Linux was capable of in 1998. I'd argue that, given their complete disregard for Tru-64 and pretty much all things DEC, Compaq was never a UNIX vendor - they just inherited a bunch of legacy systems they needed to support. They certainly didn't push for new Tru-64 based systems. Oracle wasn't a UNIX vendor at all and wouldn't become one for quite some time.
BSD sockets are also a bad example. They were the reference implementation, paid for by DARPA. The entire purpose of BSD sockets was to be copied into other operating systems. You'll notice that Linux copied them as well.
IBM and Compaq invested in Linux because they wanted something that ran on their lower-end server hardware and could handle web traffic. Oracle invested in Linux because they wanted to be the backend to all these new websites that were cropping up.
IBM, Oracle, and Compaq didn't give a rat's ass about the operating system code - they wanted the platform. If Linux had never happened and FreeBSD became the new hot thing all the online hackers were talking about, the result would have been exactly the same. They'd have poured money into the projects rather than trying to make their own thing because that's the financially sensible thing to do. The UNIX wars were over, and proprietary software lost.
Meanwhile, the last major UNIX vendor - Sun Microsystems - was giving away its own source code under the CDDL. FreeBSD ended up adopting a lot of it. That's the complete opposite effect from what you're talking about.
Sun got involved in the GNOME project and even deprecated their own CDE desktop in favor of it. Was it because it was GPL? No. It was because they saw that all the new desktop software was coming out of the Linux community, who didn't have access to CDE. Even if GNOME had been BSD licensed they would still have switched to it, because they were still trying to keep the workstation market alive at that point and CDE was quickly becoming irrelevant.
As far as I can see, the only companies interested in taking operating system code were the network appliance vendors and Apple. It only worked for them because they didn't care about compatibility.
> 1998: Many major companies such as IBM, Compaq and Oracle announce their support for Linux.
From https://en.wikipedia.org/wiki/History_of_Linux
Without big money from UNIX vendors like those, cutting down their R&D costs, Linux would not have climbed anything.
GPL was the reason why they collaborated instead of being able to assimilate the code as they were doing with BSD, like anything sockets related.
Ironically IBM has recouped its investment, now as Red-Hat owners.
That is where everything on GNU/Linux that is mainly done by Red-Hat like GNOME, Gtk, GCC, Java is being paid for.