Locktimes are currently on the order of days (roughly 3 days IIRC), which means you have days to react to a fraudulent transaction. A dedicated watchdog service being out for days at a time is pretty shitty uptime.
Not to mention that this isn't the ONLY way you can watch out for this. Your laptop, phone, or any other internet connected device can also watch for these transactions, as long as you connect to the network once every 3 days, you can handle it yourself.
>...and no watchdog will work for free.
I'm sure you are correct, but we will need to see the exact costs here. I'm guessing (and it is a complete guess) that the costs here won't be that bad at all. You could write a service that scans the blockchain for these transactions that would easily run on a very small server with a VERY large number of revocation transactions, so it's not like it's a large cost to run a service like this. Not to mention that the "fee" for these services could be paid by "penalty" forced onto the person that tried to cheat you when the revocation transaction is broadcast, incentivizing the watchdog service to catch the fraudulent transactions (since that's the only way they get paid).
Plus I have a feeling this can easily be a value-added service to many exchanges and custodial wallets.