As you keep arguing, it’s a market not a sports book. Since there are presumably non-exchange counterparties on each side of a trade, it’s unclear how an arb could bankrupt the exchange.
Unless the exchange was actually taking risk themselves. This would be an incredibly serious problem for all kinds of other reasons
As far as arbitrage…
Right now I can but DJT to win on Predictit for 48 cents. According to Polymarket, there’s open bids of 49.7 for the same contract. So in any other financial market, I’d go borrow, say, 4800, buy on the first, sell on the second, pay back my loan, and pocket $170. Classic arbitrage!
But I can’t because there’s no way to settle those trades across the two venues. The contracts aren’t actually interchangeable and this really eliminates a lot of classic arb strategies.
These aren’t real markets, so you need to be very careful about how you apply conventional wisdom about financial markets to them.