Exactly. I've worked in companies dominating a particular niche. Everything about the technology is "terrible" from an engineer's perspective, and yet nothing shifts their dominance. A massive rewrite that damages customer relationships is more risky than doing nothing to improve it.
In some fields having a good business and regulatory relationship with the customers (e.g., hospitals, DoD, banks, etc.,) is much tougher than building the product. There are many barriers in place and these fields are not so easily disrupted (having a superior product is definitely not enough).
Also note that having a bad product now that is better than the competition does not mean you cease to improve.