You are right, even if we transfer bitcoin to the wrong network, our bitcoin will still be transferred to Coinbase wallet. But the thing is, this is a rule, and they have warned and emphasized this to users before. If the user is wrong, the user is fully responsible and they will not help to get those bitcoins back even though they can do it easily.
This is the rule of most centralized exchanges for all coins, not just Bitcoin and Coinbase, and if we are careless, we will pay the price for our actions.
Yes, the only thing OP can do is contact the exchange support but I don't think they will help him since that is already the exchange policy. I have met some people in the same situation as OP on other exchanges but it seems they can't get their money back either.
Exactly, Coinbase might just ignore him because they have already given him warning that he wants to use the wrong network. Since they stated it that sending coins to the wrong network means that you will lose your coins, will give them the boldness to seize your coins. However, some exchange will not accept your transaction when you want to use the wrong network. OP, should see what he can do to get back his bitcoin but has 50/50 chance.
OP may still have a chance to get his money back but it depends. The only case I see where a user was able to get his money back, but that case was with the Mexc exchange. As far as I know, it was because the other user was lucky enough to get enthusiastic support from an exchange support staff and that staff was his friend. Also, I have not seen any other case where users were able to get their money back for violating the exchange's rules.
Furthermore, Coinbase is known to have poor support and is quite strict. That's why I think it will be difficult for OP to get support from them and get his money back.