I don't get it. What do users gain?

Seems more like it's just an incremental step in moving both OSs towards homogeneity, that's all. Just because one gets more of a look then the other doesn't mean there aren't common forces at work.

Perhaps a reflection of a work in progress as the final picture comes into focus and the sails are trimmed in the process. Raise the bridge, lower the water...sooner or later, the boat will clear and move on, and that's the real goal.

>As a developer and enthusiast <clip> there's genuinely nothing compelling

Ask investors, instead, and see what they think. Priorities, dude, priorities...

What is your actual role here? You've said "as a user" and "as a developer". You have to pick one. As a user, you can expect improved security, improved media consumption experience, improved internet experience, many more apps, sidecar, and many other improvements. As a developer, you get access to all those users who have clearly demonstrated their preference for the above and their desire to run the latest Apple operating systems. Plus, you get easier ways to port your apps to the Mac with relatively little extra effort.

If you don't see the value in either role, there are other platforms available. It doesn't get any more simple than that.

As a user, you can expect improved security, improved media consumption experience, improved internet experience, many more apps, sidecar, and many other improvements.

Improved security: Yes, except Mojave will keep getting security updates for months too and will only become less secure once Apple decide they don't want to patch it. Time will tell if the read-only system volume makes any difference vs Mojave SIPS, but it's not touted as a security improvement anyway; just a robustness issue; not that I've ever yet in my long history with Mac OS had the "system" become damaged. Maybe just lucky.

Improved media consumption experience - how so? I have no problem with iTunes and find it convenient having all that in one place, but some don't; either way, the experience is the same, no new features; it's just split into separate apps. I mean of course right now there are even fewer features in Catalina for media consumption as platlists are DOA but one hopes they will return with at least as many features in due course (my prediction though: live playlists will never come back, because iOS doesn't have them and the new split-up apps are dumbed down; hope I'm wrong, they're useful even if a bit buggy in iTunes today).
"Many more apps" - again, how so? There are far fewer apps available under Catalina - hundreds less, possibly thousands - 32-bit support is gone. Sidecar - already mentioned; you need hardware so new, some of it isn't even released yet (Mac Pro) and not one single one of my very long list of Macs will apparently support it. "Many other improvements" - such as? That's what I'm asking. You've not listed any.

As a developer, you get access to all those users who have clearly demonstrated their preference for the above and their desire to run the latest Apple operating systems. Plus, you get easier ways to port your apps to the Mac with relatively little extra effort.

As a developer, I think portware is a disaster and will continue to develop iOS and MacOS apps bespoke to take advantage of the very best each system has to offer, with user interfaces that take best advantage of the interaction and integration models they each offer. As a developer also, loss of 32-bit support is annoying. And as a developer, not supporting my Mac Pro is annoying. As a developer, "getting access to" means nothing anyway; you're just saying that somehow, as a developer, having yet another OS variant to support is good?

Sorry, but please tell me again, as a developer, apart from Catalyst what do I get? A lot of deprecations, unsupported hardware, and...? What's going to make life easier or better for writing native Mac OS apps? Have Apple decided to release Catalina with better API documentation, say? No. No, they haven't.

If you don't see the value in either role, there are other platforms available. It doesn't get any more simple than that.

Unfortunately, for anyone interested in the medium to long term health of the Mac OS ecosystem, you are entirely right.

I’ll focus on a single part of your question that I have personal experience with. The 64-bit transition. As a user, I appeared to have lost about 10 applications; all non-app-store apps. However after going to those sites and downloading the latest versions I found working 64-clean apps. Meaning I failed to update apps. Ultimately I lost 2 apps. Out of hundreds. One went out of business, the other has an update in the works. Let’s keep in mind how old general purpose 32-bit consumer use is. The early 90s. It’s over 25-years old! The biggest benefits for consumers are the larger memory access and file size footprint. For Prosumer and professional use, that’s the biggest thing. For developers, those who write 64-bit apps are no longer worrying about compatibility with archaic 32-bit libraries. Apple has warned users for a great while that 32-bit support was coming to an end. Developers as well. If you got left behind it’s unfortunate. But you weren’t blindsided. We all knew this was coming. I’ll throw in a comment on the merging of apps across OSs. I’m sceptical as well. If this is about single access, I wish Apple luck. If this is about moving off of x86, I pray they have a plan. I have yet to see a single ARM setup, production or research, that can handle being a complete computer on par with a current Mac computer.

...and money. It's all about the benjamins, baby! It is the end users' money that drives the platform, nothing else. Apple is leveraging the success of the iOS platform and applying it to the Mac. Nothing that you mentioned matters one whit. If you don't like it, you know where to find Windows, Linux, or BSD. They are waiting for you.

I think that they all mean, when talking about "more apps" is that catalina is going to be getting more apps through catalyst.