I was disappointed when I first heard about this. After reading all the posts in this topic I understand the decision and I feel more hopeful.
I'm glad developers will have more freedom. I didn't like the direction Invision was going with forcing installations from the MarketPlace in the AdminCP. I'd rather have the option of getting apps directly from the developers, but I didn't before because developers couldn't promote their websites within the Marketplace. It was hard to tell who actively supported products on their own website. I also like that developers will be able to make apps that more directly compete with other apps or restore removed features. I like that developers can quickly release bug fixes without waiting days for IPS to approve it.
I would still like a way to quickly search for modifications without visiting dozens of websites, but I think an external website can work quite well. I used to use vBulletin before they tanked, and their modification directory was hosted on a separate domain. It was an active community before version 5. I think anyone who cared about modifying their software could easily find it if they tried. That one was a simple forum, but I imagine one using IPS applications will be more robust.
As a customer, I'll continue to purchase and support apps from third-party devs. I hope that other customers will too and that the developers will be motivated enough to maintain their apps elsewhere.