If we were to take a molecule of DNA for example, and simply replace all of the carbon atoms with silicon, the resulting molecule would be much less stable and more reactive than the original. Silicon is a larger atom with more electrons that it holds less tightly. It is further a more metallic element than carbon, meaning that it is more prone to ionic rather than covalent bonding. A silicon-based abiotic precursor would be just that more likely to die or decompose which is a detriment to competitive fitness.

There is also a natural limit to cell size because in order for the reactions necessary for life to take place, the molecules in a cell must collide frequently. A silicon-based cell would be necessarily larger and therefore slower (to grow, reproduce, evolve) than a carbon-based one.