This is absolutely mad vendor lock in. I'm doing the maths and if you create the next flappy bird and it goes viral and gets 50 million downloads in a month, you'd owe unity $10 million dollars before you'd even received your first monetization cheque (you did launch with a full monetization plan, right? right? oh.)

edit: i forgot they had moneitzation limits too, so no - this situation wouldn't quite happen until they earned $200,000 in revenue. Though the potential to go viral and find yourself underwater because of the massive unity bill in comparison to your income is still a possibility