"probably won't steal" is more accurate. 

you *will* have a channel open with the ACINQ node. is your Phoenix node always online? are you using a watchtower? your own or from a service?

i am not saying ACINQ is going to steal channel balances. they are incentivized to support their customers.

using Phoenix is entering into a banking business partnership with ACINQ. they are making decisions about how to handle your payments, and you are paying them to do so.