There are many rumors behind this decision, from a supposed deal with Adobe, to more simple things like cutting costs. Personally, I don't believe the last option, but who knows ?
So, what's so great with Aperture ?
It's rock stable, period. Everything as tested to exhaustion. I can't remember about a single bug in it.
It's fast. Runs well on old computers and not a memory hog.
The Digital Assets Management (DAM) function is indeed extremely good. It can handle large image collections and organize them in projects, catalogs, and albums. The rating system is near perfect, with color tags, star rating, keywords and other flags.
The searching tool is by far the most powerful one ever implemented in any DAM of my knowledge. It's possible to search by all sorts of parameters, alone or combined.
I tested it with libraries having more than 80K images without any problem. It's possible to use more than one library and to move images between them.
It's possible to consolidate all images inside the library itself or let them outside, just referencing them to the internal database.
The image editor is very nice if you don't need to work with layers or do cut-paste-make-boobs-larger and such things, it's a good tool for photo adjustments, like colors, tones, retouching, levels, etc. The retouching and cropping tools are simply perfect.
The raw engine is part of the operational system and it's updated according to with Apple's policy to the OS you have installed. It includes literally hundreds of camera models and the updates come as an OS update.
If you need, it's possible to use an external editor with Aperture. I use Affinity Photo for this.
There are many interesting plug-ins compatible with Aperture. My favorites are:
The Nik Collection:
Analog Efex Pro: Simulates film/vintage look
Note: Until mid-2018 the Nik Collection was owned by Google and available as freeware, but DxO acquired it and now it's paid (and expensive) and I'm not sure if the new version is still compatible with Aperture. If you have the old free version, keep it safe !
DxO Film Pack: Professional film look simulation
Noiseware Professional had an Aperture plug-in in the past, but it was discontinued. It was a wonderful professional noise reduction tool. If you have it, grab with both hands !
The later versions of the RAW engine can handle fairly well the Fuji X-Trans raw files. Much better than Adobe Camera Raw.
Actually, with the advent of Affinity for Windows and all the important photography software being ported to Windows and the fact Apple is again making Mac user's life miserable again due to soldered memory and SSDs, making Macs not user upgradable or serviceable, I see no point to having a post-2018 Mac. I don't like Microsoft Windows, but I like even less the new approach from Apple.
Macs are becoming just too expensive and a potential headache along time.
Some of the cameras supported by the High Sierra raw engine aren't supported by Aperture. Surely Apple did this on purpose to force people to use Apple Photos.