Nobody's come even close to matching Deck's price point for a portable PC with an HDR OLED 90Hz screen. Spending more to get less in return has always been outdated.
I'd argue the current Ally does a decent job of putting up a fight if you get it on what appears to be a pretty regular sale. I've seen it new at £530, with great condition pre owned ones going for £400 or less. Compared to the £479 starting price of the OLED deck I honestly would call it a fairly even game that comes down to what you want out of your handheld.
Steam Deck clearly wins out in a more streamlined console experience out of the box, and is also the choice for gaming away from a plug socket. There's a caveat that you won't get quite the same performance compared to something like the Ally, but you're also likely targeting a lower resolution as standard on the Deck anyway.
The Ally has power on its side and is what I would consider to be a more portable package. The Windows support can be a plus for some and despite it taking more to setup out of the box, can still stand up as a gaming system. You lose out on battery for sure (which is why I'm hoping the improvements on the Ally X are substantial) but as somebody who's rarely away from a plug, even when traveling (most trains here have plugs on them, which is nice) I really feel like I can make the most of the Ally's performance.
I also feel like they're on pretty even grounds in terms of screens. The Deck's HDR OLED 90Hz screen is obviously lush, but I do personally prefer the VRR of the Ally's 120Hz screen. Even as an LCD it looks great, so I do think both have their perks.
I'll never deny that the Deck is a stellar pick for a handheld, but I do disagree that it's the answer to every portable PC gaming problem like everybody makes it out to be. I want to keep seeing more options, from both Valve and competitors to really drive the space forwards more.