DEC basically ended up getting commoditized by Linux; they still had momentum when moving from VAX to Alpha. In the early 90s they were able to migrate many of their VAX/VMS users to Alpha/OpenVMS and their Ultrix users to OSF-1, but by the end of the 90s, many of their ISVs had released Linux versions of their products. SGI at least had niche applications that let them hang on independently for a bit longer.
DECs offerings really couldn't compete with that, and there was noting Compaq/HP could do about it.
Haven't the same happened to SGI, DEC(VAX), and OS/2 itself? All much bigger successes at their time than Symbolics has ever been.