I gotta disagree. The only time that extra horsepower is gonna get any play is when you're using it docked, because playing games at 1080p while portable pretty much guarantees less than an hour of battery life. For indie and 2D games the lower resolution does not matter. Windows on a portable is a less-than-pleasant experience, and the price difference isn't negligible either. I think Valve struck a better balance with Deck's design.
I'd rather play at 720p with higher framerate/graphics settings. No issues there. Even on an Ally, I'd still play on 720p (or use AMD FSR)
The issue is more with demanding games. Of which there are quite a few on PC, and upcoming/future games are going to be even harder to run, meaning if you are someone who likes to play the newest demanding games the Steam Deck is going to become obsolete very quickly, even if it isn't quite yet.
The additional power of the ROG Ally is not just good for running games at 1080p. It gives you way more flexibility in how you like to play games. You can push the graphics settings higher if you don't care about battery life or playing tethered to a wall, you can use the same graphics settings/framerate and enjoy increased battery life due to the improved efficiency of the SoC in the Ally, or you can simply enjoy improved framerates. You aren't just stuck on low settings and 30 fps or sub-30 fps in demanding games like on the Deck. From what I have seen the difference is significant enough that 30 fps games on Deck can run closer to 60 on the Ally, and that makes a big difference.
It also has a superior cooler system that runs cooler and quieter, which from what I have heard also keeps the heat away from where your hands are better, improving comfort.
I don't think anybody who cares about battery life buys one of these things because that is their weakest point. Playing tethered to the wall during longer sessions is pretty much a fact of life that anybody who uses these things has to get used to. Which generally turns out not to be much of an issue because these devices are largely targeted at the person on the go, who doesn't have much free time to sit down and play at home, they are the ones who get the most use out of a portable since it allows them to use those short bursts of time they have during breaks, or on the 30 min commute to/from school/work to finally get some gaming done that they would otherwise never have time for. And I don't see a 1 hour battery life being much of an issue for that type of person; it's enough for them to make it to school/work and put it on charge so that it's fully charged by break time. In lighter games 1080p won't have a big effect on battery life anyways.
But let's be real, these devices are not made for playing indie and 2D games, those are almost always on Switch and a Deck or ROG Ally doesn't give you any advantage performance or graphics wise, just increased bulk, weight and fan noise. Sure, people play indie games on them too, but that's not what people are buying them for, the whole appeal is the promise of a more powerful handheld (compared to the Switch) capable of running AAA games, which the Ally just does better.
Granted, Windows is not the ideal OS for a portable, but you don't have to run Windows, that's the beauty of it being "just a PC". With Steam Big Picture being updated with the Steam Deck UI, you can run any Linux distro of your choice and get the Steam Deck experience on anything (or use something like ChimeraOS which is a sort of unofficial Steam OS), and I've heard of people putting the Steam Deck OS on various devices as well including the Ally.
As for the price, the MSRP difference compared to the 512 GB Deck (which is the closest comparison) is only $50, which is more than fair considering all the improvements, I don't understand why people keep bringing this up like it's a bad thing.
If you have a base model Steam Deck, the ROG Ally may seem like a big price difference... But you could conceivably get away without a large MicroSD on the Ally, I am sure nobody uses the base model Deck without one, so that's an added expense that needs to be taken into account too (and MicroSD storage is much slower than a SSD, so even if it is cheaper, it's not ideal for games with long loading screens)
Admittedly, even with the required cost of expanding the storage, that $399 base price is a lot easier for people to swallow and nothing else on the market can come close to matching that price, so the Deck has a big advantage there. I just don't personally think the base model is a good purchase unless you intend to upgrade the SSD with a 1TB/2TB one yourself (in which case by all means go for it, you get a better deal compared to the 512GB model
and more storage), but many people just want to buy something and not have to fiddle with hardware upgrades themselves, and the 512 GB is almost definitely going to be the best size for those people.