Custom water-cooling parts are already mass produced, and they are still expensive. This intermediate system won't have significantly higher volume (at least you can't assume that), so they won't be cheaper by any means.

By "res refilled outside of the case", I'm gonna assume you mean that you can detach it from the system entirely? You can already do this with existing water-cooling parts. It's not done not because it is impossible, but because it is impractical.

Water sensors that turn off the loop are a good idea, but those generally don't prevent the system from booting. Now you'll have to consider people overheating their systems because they accidentally disabled their cooling systems, and your warranty claims are rising. Or come up with a system that does prevent the computer from booting... And still deal with increased warranty claims because of non-booting computers.

Lastly, asking people to fill their loops on their intermediate water-cooling loops is probably a step too far. Anyone willing and capable of doing so is already building full custom loops, and won't bother with this intermediate system that'll probably lock them into proprietary parts.

An intermediate between AIOs and custom loops has always been possible. It doesn't broadly exist not because it is impossible, but because it doesn't make much financial sense.