Not one critic has put up a valid alternative. Except maybe the old LG 5k.
Show me a bright (500+), well built and supported (not cheap plastic without support), high PPI (210+), 27+ inch display as a competitor to the Apple Studio.
It's a bit cheeky to draw up a list of specs and expect that what constitute a "valid alternative"
must have all of these features, because otherwise I could do it for absolutely every single display in existence.
As in "Show me a 42" 4K TV with a local contrast ratio of at least 8000:1 that supports hardware calibration via Calman, Dolby Vision, HDR10, HLG, VRR, 4 HDMI 2.1 ports" and bam just like that the list of "alternatives" suddenly gets very small.
It's similar to when public administrations draw up tenders with a set of requirements that "just happen" to coincide with what a single company in particular can produce...
The ASD's spec list is very unique indeed but that doesn't mean that there's no valid alternative elsewhere for a lot of people as long as compromising on some of these unique specifications is a possibility as a trade-off for other benefits.
What I do like a lot about the ASD is that just like with the iMac, the spec list is a pretty decent balance for a wide range of applications, even if it means not being superbly appropriate for any one of them. That sort of balance makes a lot of sense for an integrated display, and probably as well for MacBook users who want a single, larger display that does it all on their desk when they're back home, or for ex iMac 27" users who no longer have that option from Apple, for example.
But that's also where marketing it as a companion to the Mac Studio is a bit weird. The latter's users might be a bit more selective about the features they want / need in an external display and the catch 'em all features list might not be quite as appropriate in their case.
an HDR display with a brightness of 350
Specs like HDR400 [...] on a computer monitor is truly relevant - as a bad monitor.
Maximum brightness aș specificed on a spec sheet is a poor predictor of the quality of the HDR experience you'll get.
VESA's HDR 400 certification is a total joke that misleads consumers, but in a darker environment, with quite a lot of content, you'll get a better HDR experience with that :
https://www.lg.com/us/monitors/lg-32ep950-b-oled-monitor
than with the XDR display, even though it only reaches a paltry 250 or so cd/m2 full screen.
That's what VESA's HDR 400
True Black certification is for

.
Apple's marketing for the XDR display isn't much less misleading in regards to its actual HDR capabilities.