Let's not forget that should you decline an offer, the other party has options like shifting to hypercompetitive approaching anti-competitive tactics: undercutting you to bleed you dry, burying you in frivolous lawsuits, propoganda campaigns against your product, and so forth.
The list goes on and many different tactics are perfectly legal or can be ambiguous requiring proof of motive (nearly impossible). You're engaging in war with someone who has far more resources than you. Sure, you couod win and shift or supplant their market share but more likely you'll be crushed.
Choosing between that option and $50m retiring happily, I'll choose the second option every time. I like the idea of competing but let's be realistic, this is not your restaurant competing with the other guy down the street where it might really come down to your actual competitive advantage in what you're providing a customer. Many times it comes down to business strategy and resources to execute those strategies.
$50m realized, now... cash in your pocket vs. maybe $1B.... but you don't know - there's a very real chance the company will die before that.
50m now vs 1b later is way too simplistic of a view.