[rmk]

> rewrites your codebase from scratch

This almost never happens. It takes a long time and huge amounts of money to come up to parity, and in the meantime, the legacy org is earning money on the thing you're trying to rewrite.

It's more often the case that the technology landscape shifts dramatically helping a niche player (who has successfully saturated the niche) become mainstream or more feasible. Take, for example, Intel. Their CISC designs and higher power consumption is now being challenged by relatively simpler RISC and lower power designs. Or Nvidia with its GPUs. In both cases, it's the major shifts that have hurt Intel. No one can outcompete Intel in making server CPUs of old, if they are starting from scratch.

Take another example, this time, of a successful competitor (of sorts). Oracle vs Postgres. Same deal, except that Postgres is the successor of Ingres (which doesn't exist anymore), and was developed at Berkeley and was open-source (i.e., it relied upon the free contributions of a large number of developers). I doubt that another proprietary database has successfully challenged Oracle. Ask any Oracle DB user, and you will likely get the answer that other databases are a joke compared to what it offers.