MangoTBG is correct, the GPU market would be almost stagnant, but not quite.
Consider Microsoft and their monopolies in desktop OSes, Office suites, and browsers.
Development on browser features slowed way down but didn't entirely stop as there have been minor additions to attempt to add a server OS monopoly to the family.
Office suites, MS adds a sniny new look every couple of years to make you think it has changed, plus a few new features so it looks like your old copy of Office is missing something.
Windows itself gets the shiny new look, plus bundling of new apps in whatever areas MS thinks it might be able to leverage its OS monopoly into a chance to gain dominance (for example, DRM protected audio and video).
See, monopolies aren't all bad, nor do they mean an end to "innovation"!