They are both a marriage of technology with quite different economic systems. With different pro's and con's.

But a technological/economic system that highly incentivized suppliers of fine grain pick-and-choose modularity (in the form of both modular systems and individual modules), over bloat/irreversably-integrated products would be an incredible boon for choice.

Because modularity = choice, maximum competition. Non-modularity = lack of choice, reduced competition.

Then vertical proprietary systems could still compete, including leveraging integration when it provides benefits (performance, simpler choices, lower user-side complexity), but they would be competing with viable modular substitutes with a different benefit: they do what users want, and only what they want.

Not saying I have a realistic economic model for how a very strong modularity incentive happens, but the question has been on my mind the last couple decades!

Note that both proprietary and open-source provide haphazard modularity, but neither is pervasively modular today. Although the latter is reliably malleable, sort of atomically modular. But we the option for modularity at all levels clearly has huge untapped/unrewarded economic value.