You do not loose any control the TA is your agent and acts for 'you' they do have more clout, if they put x% of business DCLs way and are multi cruise line then they can direct business to another line.
DCL is well aware of the importance of TAs hence paying good commission and having offers for TAs to cruise with them or look and see or being invited to the last launch events of the newer ships.
I have recently had an issue on my reservation at first DCL said they 'cant do it', my TA said do not worry and had sorted it out by Monday morning speaking to someone high up.
Like a work 'union' collectively for the business they have a lot of power, I am not saying Costco here I am saying good specialist DCL TAs.
Another example goes back a few years, I booked Palo tea, it disappeared, when I re checked Tea was sold out, my TA got it back for me. Yes a small example but they have power.