I'd argue that the Mac developer community was hollowed out by mandatory Sandboxing in the Mac App Store, introduced in 2011, not by the iPhone (2007). Sketch, arguably the most successful third-party Mac-only app since OS X, was released in 2010, several years after the iPhone, but almost immediately before mandatory sandboxing.
I believe the reason sandboxing destroyed the Mac software ecosystem is because it's too restrictive to support the kinds of productivity utilities that have historically thrived on the platform. And those apps are too small to handle promotion without Apple's help, Apple's help now being reserved for apps in the Mac App Store, which are by extension sandboxed.
Sketch is an interesting one, gained so much traction for having a superior interface but after several years in the lead it's getting devoured by a cross platform web app with superior collaboration.
It's relevant that has also been essentially 10 years of stagnation on the frameworks that Sketch is built on, i.e., AppKit, as Apple focuses on UIKit. In this case specifically Apple hasn't added best-in-class collaboration features to AppKit (or UIKit). I'm not sure whether it would have made a difference in Sketch vs. Figma, but the fact that they haven't has made it difficult for Sketch to compete, as Sketch's MO is leveraging Apple's built-in technologies.
On one hand I think macOS sandboxing can be a bit too restrictive and buggy, but on the other I think it's somewhat unreasonable for developers to continue to expect unfettered access as a given on the desktop, because it's been proven time and time again that unfettered access will be abused, often by big companies that users tend to implicitly grant a higher level of trust to.
I agree 100% in principle, the problem is Mac App Store apps compete with non-Mac App Store apps that have unfettered access, and macOS itself competes with other OSes where unfettered access is expected.
>I believe the reason sandboxing destroyed the Mac software ecosystem is because it's too restrictive to support the kinds of productivity utilities that have historically thrived on the platform.
And yet, MS Office, Lightroom and others are on the App Store.
I'm not quite sure what your point is, but those are 100% the most interesting cases of apps conforming to the Mac App Store that I wouldn't expect to. Here are my explanations:
1. Lightroom is easy, Lightroom Classic is not in the Mac App Store and is still supported. Anecdotally Lightroom Classic appears more popular than Lightroom, especially among professional photographers where it's fair to say Lightroom is shunned. So Lightroom is Adobe's attempt to have it both ways.
2. Office is much more interesting. I honestly find it shocking that Office is in the Mac App Store, since I don't think it's in Microsoft's best interest. I think the reason is that the Mac versions of Office are really stripped down and separate code bases than the PC versions, and I'd guess that this decision was in some way made possible by that separation (e.g., if there's a feature that's not possible in sandboxed version, it can just be stripped from the Mac version, which would be more difficult with a shared code base). I'd also guess that this will eventually be reversed as Microsoft is on a path to making cross-platform web-based (Electron/React Native) versions of all their software, and I don't expect those to be sandboxed.
For the record, by productivity software, I didn't mean things like Office and Lightroom (my fault for not being clear). I meant utilities like LaunchBar, Keyboard Maestro, Moom, etc... e.g., launchers, window managers, and the like, which generally need more privileges than sandboxing allows, because they involve scripting, interacting with other apps, etc... A lot of the indie Mac app community centered around these apps because they allowed users to get a lot of value without much code (relatively) by adding smaller, system-wide, features.
Document-based apps work much better with the Mac App Store, but those apps tend to be an order of magnitude more work to make, therefore there were always much fewer of them. Document-based apps also have a lot of problems with Sandboxing, but at least they're possible to make sandboxed.
I believe the reason sandboxing destroyed the Mac software ecosystem is because it's too restrictive to support the kinds of productivity utilities that have historically thrived on the platform. And those apps are too small to handle promotion without Apple's help, Apple's help now being reserved for apps in the Mac App Store, which are by extension sandboxed.