(Repurposed email reply)
Although I was educated & worked with DEC systems, I didn't have much to do with the company itself. Its support was good, the kit ludicrously expensive, and the software offerings expensive, slow and lacking competitive features. However, they also scored in some ways.
My 60,000' view:
Microsoft knew EXACTLY what it was doing with its practices when it built up its monopoly. It got lucky with the technology: its planned future super products flopped, but it turned on a dime & used what worked.
But killing its rivals, any potential rival? Entirely intentional.
The thing is that no other company was poised to effectively counter the MS strategy. Nobody.
MS' almost-entirely-software-only model was almost unique. Its ecosystem of apps and 3rd party support was unique.
In the end, it actually did us good. Gates wanted a computer on every desk. We got that.
The company's strategy called for open compatible generic hardware. We got that.
Only one platform, one OS, was big enough, diverse enough, to compete: Unix.
But commercial, closed, proprietary Unix couldn't. 2 ingredients were needed:
#1 COTS hardware - which MS fostered;
#2 FOSS software.
Your point about companies sharing their source is noble, but I think inadequate. The only thing that could compete with a monolithic software monopolist on open hardware was open software.
MS created the conditions for its own doom.
Apple cleverly leveraged FOSS Unix and COTS X86 hardware to take the Mac brand and platform forward.
Nobody else did, and they all died as a result.
If Commodore, Atari and Acorn had adopted similar strategies (as happened independently of them later, after their death, resulting in AROS, AFROS & RISC OS Open), they might have lived.
I can't see it fitting the DEC model, but I don't know enough. Yes, cheap low-end PDP-11s with FOSS OSes might have kept them going longer, but not saved them.
The deal with Compaq was catastrophic. Compaq was in Microsoft's pocket. I suspect that Intel leant on Microsoft and Microsoft then leant on Compaq to axe Alpha, and Compaq obliged. It also knifed HP OpenMail, possibly the Unix world's only viable rival to Microsoft Exchange.
After that it was all over bar the shouting.
Microsoft could not have made a success of OS/2 3 without Dave Cutler... But DEC couldn't have made a success out of PRISM either, I suspect. Maybe a stronger DEC would have meant Windows NT would never have happened.