It's rarely the case that there's only room for one player in a given market. Market segmentation is a real thing, for good reasons. If $BIGCO pivots to producing a product similar to yours, that doesn't
have to mean "the end". I'd suggest everybody read
The Discipline of Market Leaders for more on how to choose among different strategies and position yourself vis-a-vis competitors.
Of course the problem of fundamentally building on someone else's platform remains. This is why it's a good idea to start at least thinking about how to move your business onto another platform, from day zero. Of course there's a balance between investing time and money in building abstraction layers and evaluating other platforms and doing these different things, versus spending that time and money on immediate growth. But if one invests literally 0 in this situation, they can hardly expect much sympathy if the underlying platform gets yanked out from underneath them and kills their business.