Which means you're trying for trad-pub first. When I was trying this path I could not find that many representatives in Australia that matched my sort of work, and I decided to draw the line at seeking representation overseas - so to some extent I am jealous.
Now watch as this post wavers back and forth between sides of the argument...
The fact that you've found someone that will even talk to you about your book is a very positive thing. My limited experience is that the vast majority don't even bother to acknowledge receipt. So don't throw away this opportunity if you can avoid it, but don't sell your soul either.
Sometimes we know something is wrong, we just haven't had the experience/knowledge/courage to deal with it. So if you get reliable advice that helps you fix a known problem, go with it.
But be wary of making changes that feel wrong to you. Look to see if the complaint/suggestion actually indicates a different problem; people are generally good at recognising there is a problem, but even professionals can be wrong about how to fix it. Consider that the next agent you go to may have the reverse opinion (fiction can be like that). These people are professionals (assuming you found someone with a good reputation), and so their opinions should be taken seriously, but they are still just opinions.
If the advice feels wrong, seek another opinion from someone you can trust to tell you if you are being overly protective. Maybe what you're feeling protective about just isn't that important, and maybe you need someone to tell you that.
You say the agent speaks of "sermonizing". I'd take this seriously, it's something readers can be quite sensitive to. So far you only suspect the agent is going to require annihilation, I'm more inclined to think that what they want is to have it cut back to its core. With some of this sort of stuff, a little can go a long way (as a reader, I don't necessarily have to be reminded of his pomposity and/or verbosity with every speech he makes).
This is very personal stuff, so no one can tell you how to handle it. So much depends on your own priorities. If publishing success is your priority then I suggest placing additional emphasis on advice you obtain from professional quarters like this agent, as they are the ones that have some idea what is likely to sell. If publishing
your story is the priority then you need to remember that self-publishing is an option if you cannot persuade someone else to take the story as you want to present it; you still have the option of paying for your own editor and cover designer and so on to get the job done properly.