They hold 1 key and you hold two. Two keys are needed to spend. Technically, they hold two keys, but one is encrypted with your password. So, in theory, they can't spend your coins, and you can spend them without them. It looks pretty much the same as Bitgo.
So what is the point of coinbase multisig ? When you lose one of your two keys then you could still spend your coins with coinbase help, thats it ? But it is a question whether coinbase or other service like BitGo going to be around in 10 years...
If you plan to dont touch your Bitcoins for so long then paper wallet seems best option to me. Hard to imagine Bitcoin value after so long time. Do not encrypt the paper wallet so the Bitcoins are not lost when you lose the passphrase after so long time.