Judging from the response on this site (no one has said they prefer to have a travel agent plan it), I wouldn't think travel agents would have much business. But I think that is just the nature of people on this site.
I can't imagine someone who has a travel agent plan all of their trips contributing to this site. What would they say when asked for advice?
In my case, I love the planning. I maybe enjoy the planning more than the trip itself. I have plans already for trips I might never take. I just planned them because it was interesting.
A few years ago, in June, my wife said, "Let's go to Germany for Christmas." Within two weeks, I had everything arranged. I had been thinking about my next trip for a year, planning on where to stay, what to see, how to get between places. All I had to do was make the reservations.
I really don't think a travel agent could plan my trips. I don't use hotels that pay a commissions to travel agents, so they probably wouldn't pick those places, and it would cost a lot more. I don't think they would have access to the discount rail tickets or Länder tickets that I use. They would probably want to sell me a rail pass. I always compare using a rail pass to the way I plan on getting tickets, and a rail pass always costs more.
A few years ago, I signed up for a community college course called "How to plan a trip to Germany". It was "taught" by a travel agent who brought in the broshure from a packaged trip to Germany she was trying to sell. She had no tips on planning a trip, except to buy her package. A few of us demanded and got our money back.