While you can get trimmers that use blades, they are obviously no longer string trimers at that point, even if they came from the factory with a string head. Notably, they have considerably more risk when encountering a solid object, or you - depending on blade design, blades can break and fly off, or fail to yield when encountering an obstruction and cause the machine to kick wildly.
Depending what and how much you are trimming, all the pre-string-trimmer options apply - grass shears, hedge shears, powered hedge trimmers, scythes, sickles, etc. and they may be safer than some string-trimmer metal blade options. String trimmer plastic blades still shed plastic waste as they wear, and become plastic waste when they lose too much of (at least) one blade.
There do appear to be (supposedly) biodegradable trimmer line products showing up on the market - that should have been a priority from the outset, but obviously was not.