One doesn't reduce the use of plastics without using something else as a replacement.

If we replace all plastic cups with glass cups, might we get micro-glass inside ourselves instead?

Glass, mostly silicon dioxide, might sound harmless, but glass can chip off in microscopic sharp fragments (think fiberglass dust - which some suggest might be as bad as asbestos). Glass also slightly dissolves in water (this is how glasses start to look frosty after enough dishwashing - they're dissolving), and slightly evaporates (everything has a vapour pressure!). And obviously the glass isn't pure - there can be all kinds of deliberate and accidental additives.

Am I worried about glass? No. But it seems naive to wholesale replace plastic with something else until there is a decent understanding of the replacement.