Adobe have successfully built a moat around particularly Photoshop that no one seems likely to get close to crossing.
At home I use mostly Affinity, at work it's Adobe all the way.
Photoshop is unique for the many professions it supports: web designers, graphic designers, photographers, digital painters etc. If you are looking for a PS replacement, first ask what you are going to use it for. Other apps may be the more natural choice.
God is in the detail. I would hesitate to use Affinity photo for digital painting simply because there is no slick way to change the flow rate of brushes. Recent changes have improved this, but not to the point where it is comfortable to use.
Affinity Photo is not Photoshop. That said, it's a great image editor in and of itself. Making direct comparisons I feel is unfair, yes it is glitchy and can, and does, have inconsistent UI behaviour though is only occasional in my experience.
But for US$70 per major version, perpetual (no subscription), for the small team behind it, compared to the Adobe behemoth behind Photoshop, it's pretty damn good.
As of writing (2024-06-08), Serif's entire suite is 50% off at the moment, which makes Affinity Photo US$35 perpetual, for (in my opinion) 80% of Photoshop's capabilities, when Photoshop itself is US$22.99 per month (individual subscription).
Designer blows Inkscape out of the water. Yes, I know I'm comparing a commercial package to an open source package, though for professionals who don't have time to troubleshoot bugs in open source software, let alone time to write their own patches, Inkscape sadly doesn't cut it here.
For what its worth, I am a huge advocate of open source software, though I'm also a professional that needs my tools to work now, not in six months when someone gets around to helping/assisting/writing a patch.
Downsides for Designer though, it does lack some features, like image tracing, though it's not a deal breaker for US$70 perpetual, for a user interface that's intuitive (sorry Inkscape), and that spits out SVG images that don't cause problems in other consuming applications, or SVGs that fail to display correctly.
Coincidentally I do use Inkscape for image tracing though migrate over to Designer after that.
As much as it sucks, the only way Serif will get close to knocking Photoshop, or Illustrator, or InDesign, off the 'industry-standard' pedestal is if we all support them by kicking in a few dollars for a licence, even the cheaper iOS tools, which are more-or-less on-par with their desktop counterparts.
Likewise for open-source software: they'd easily start winning over people with a better UI design (again, in my opinion, most FOSS software in this space feels unpolished) and perhaps commercial sponsorship like Blender has been able to partner.
Before version 3.0, Blender's UI sucked and drove a lot of people away. It's now easy and intuitive, if overwhelming with the number of features it has, attracting a lot of newbies and kids wanting to learn 3D.
Blender (there are some really big names here!) - https://fund.blender.org/
Inkscape - https://inkscape.org/*sponsors/
GIMP (Web Archive as GIMP's website is having cache issues right now) - https://web.archive.org/web/20240603082716/https://www.gimp....
(I have no financial interest in Serif at all, apart from owning licences for their software.)
I (a non-designer web engineer who has absorbed some skills in thirty years of hanging around a world class designer) really like Designer and would never buy Illustrator, but I really admire Inkscape.
Just as a side-note because I enjoy recommending a weird app every now and then, have you ever played with NeverCenter's CameraBag?
It looks, on the surface, like a cheap toy app. But appearances are deceptive. The filter chain tool is wildly impressive, and some of the colour curve tools are unique, getting you almost towards LUT creator tools in terms of their power.
It's not something you'll use every day but it's very interesting for visualisation, and it makes some things so easy for newbies it's worth knowing how it works just so you have a tool to recommend.
(people get annoyed when I share my full views on the way GIMP is an oft-recommended open source "alternative" app, so I won't)