Bringing back an cruise desk and an actual person to talk to would allow people to ask questions and the opportunity for them to explain things better than just a simple 4 step process in an app.What would bringing back the future cruise desk do for you in this situation? Sounds like the issue is that you didn't understand your TA's policies, not that there wasn't a future cruise desk.
I'm sorry you had a bad experience with your TA (though if that's the case I'm confused on why you said you wanted to work with them again when you bought the placeholder?). There are plenty of good TAs out there. It's your choice to work with a TA or not, but you experience is not reflective of all TAs. It's not clear to me why you don't want to work with a TA, but perhaps you can try a different agent within the same agency?
DCL doesn't need to remove the 'assign to current TA' choice because it's just that: a choice. If that happened automatically then I would agree. But people are free to choose how they want to handle their placeholder, and if it saves a step by being assigned to the TA at the time of purchase, then why not. You don't have to do it. Just like with purchasing the placeholder itself, it's on the buyer to be aware of all policies related to that purchase/transaction.
And there's no need for a cooling off period; this is not at all like buying a car. I get that you're upset; however, it's not DCL's fault as you had the choice of how to handle the placeholder, and it's not the agency's fault for following their policies.
As for the TA, I did not have a bad experience with the TA they were fine. Being an experienced cruiser but a noob when it comes to using a TA I found I am more comfortable being in control and doing the planning and reservations myself (everybody is different and TA's are the preferred way's for some, just not me). Now that I have discovered the hard way that I am no longer in control of my reservation if I assign it to a TA is even more reason why I prefer to keep it myself.
I agree that the choice to use or not use a TA should remain in the app for like you said it saves a step or two for those that prefer a TA, I would suggest that DCL modify the app in some way to make the decision more deliberate and to explain the ramifications of each choice. Something along the lines of If you choose "yes" the TA owns the placeholder and controls it, if you choose "no" you have 30 days to assign it to a TA if you so choose, etc... This is information a person is better at explaining than in an app.
I do not blame DCL in any way other than to suggest they could refine their app (or bring back the desk) to help future cruisers. My angst is with the TA, if they have invested time and resources into a reservation at which point I fully understand the need for said policy. But for a situation where they just received an email and have not invested any time or money into the reservation, it would make better business sense to work with the customer rather than to outright say "no" resulting in discussions like this on boards.