It's not necessarily as easy as SP vs TRAD, either. I'm a 'hybrid' in that I have an agent, and a contract with, but no print publisher. So the e-books and paperbacks are on Amazon and Createspace.

My agent shopped 'Legion' around in the normal way, and got no bites except the Audible offer. Eventually we agreed to accept Audible's offer and SP through my agent's private label publishing arm.

So was it worthwhile having an agent? Oh, hell, yes. He's negotiated 3 foreign-language deals for me, negotiated 2 follow-up contracts with Audible; he's vetting speaking offers, his office has filed several DCMA notices for me... Don't ever make the mistake of thinking that all an agent does is shop the book around. That's just the front hallway--there's a whole houseful of services.

As to the trad-pub situation, we did get a big-five offer on my latest book, but turned it down. First, because Audible's offer was better, and second because the TRAD royalty % was lower.

Trad pub is still a goal, because it can increase your exposure, but I don't think it'll necessarily make you that much more money. Of course, YMMV. There are enough variables in the whole publishing thing that everyone's experience can be unique.

As to brick-and-mortar book sales declining: yeah, probably, to a certain extent. But the number of titles available has increased dramatically, so a 'drop in market share' doesn't necessarily mean a drop in business, just a same-size piece of a much bigger pie.