OpenWRT is so fucking easy to install and configure (easier than some consumer out-of-the-box experiences, even) that there really is no excuse if you expect a secure local network ... there is no actual technical knowledge required, just basic keyboard/mouse skills, and reading comprehension.
I think you're *wildly* overestimating the skill and confidence of the average home network user and the quality of open source project web sites. Let me walk you through the hidden minefield in your instructions. I'll use a Linksys WRT150N for reference.
The real Step 1 is "realize that I'm supposed to install OpenWrt, and understand what that means". Most users have little to no idea of how the router actually works, so the idea of upgrading the firmware is not an obvious one.
But let's say someone tells them to do it. They go to the OpenWrt web site. The second sentence under "What is OpenWrt?" is "Instead of trying to create a single, static firmware, OpenWrt provides a fully writable filesystem with package management.". Many users will be too terrified to proceed beyond this point. But let's say they make it to the Table of Hardware, and skip past the text about developer snapshots and hardware VLANs and the note from 2009 saying that the page might not be up to date. (That's not realistic -- many users expect to read sequentially.) Instead of a column that says "yes, this router is supported", there's a column named "Status" that gives the first OpenWrt version that supports the router. Next to that there's a column named "Version" that is undefined. I'm assuming it's the router version, but many users could get confused. But the important column is the "Target" column, which lists the specific OpenWrt platform that users should (but probably won't) remember for later. There are two targets for the WRT150N and no indication of which to choose. One of them no longer exists in the current version.
Clicking on the model number in the table gives me an unorganized series of notes from various users. One of them, "An account of flashing OpenWrt to a WRT150N", sounds sort of like installation instructions, but is too brief and technical to be of any use. It does have a working download link, but it's to a version that's five years old. The one after that suggests that one target option (the nonexistent one) is better than the other. None of this is in clear newbie-friendly language and it's all after pages of Linux log dumps. If they land on this page, most users will probably click the back button as fast as they can.
Alternately, we could do it your way:
Step 1, find out what runs on your router (at wikidevi or similar)
That's somewhat better, but they still have to read through a dense, abbreviation-heavy table of technical specs. (That's after they figure out they need to search for their router's model number and not "Linksys".) At least there's a simple indication that OpenWrt supports the router. But how would they know to go to WikiDevi? I hadn't even heard of it before today. And most importantly, how would they figure out which target to use, or even that targets exist?
step 2, download the firmware image
Now we're in for some fun! There's a download link at the top of the OpenWrt site. Clicking on it gives me a directory listing. None of the directory names look like they contain software to download, even to me. On the right side of the OpenWrt main page there's another download link for the latest release. This gives another directory listing. (Apparently the correct directory is /attitude_adjustment/12.09.) Now there's a list of subdirectories that look (to me) like part numbers for embedded SoCs. These are the targets. None of them say "Linksys" or have the router model number, so most users will be hopelessly lost here.
But wait! There's a README! Not a README.txt, so they'll have to tell Windows which program to open it with. No problem, let's try Notepad... oops, it's in Unix format, so there are no line breaks. If the user manages to open it in Wordpad, they'll find a recipe for a mixed drink (the aforementioned "attitude adjustment") and... nothing else of use.
The OpenWrt site also has a Beginners' Guide. It starts by saying that OpenWrt is very complex and that installation is highly context-dependent. After that it sends the user back to the Table of Hardware and the download directory. There is, however, one page with actual instructions: "The general method for picking the right image is visiting the Table of Hardware, then matching the 'Platform & Frequency' of your device with the image you're downloading. Sometimes, the 'Status' column will have additional information for your device, such as building its image or device-specific installation instructions." Hallelujah! Of course, the "Platform & Frequency" is two columns, and only "Platform" matters, and there's no indication that "Broadcom 4704" will be abbreviated to "brcm47xx" (as in the Target column), but it's a start.
Assuming that a traumatized newbie manages to stumble into /attitude_adjustment/12.09/brcm47xx/generic/, they're confronted with over two dozen files plus another freaking subdirectory and WAIT IS THAT MY ROUTER'S MODEL NUMBER OH MY GOD SOMETHING FINALLY MAKES SENSE IS THIS WHAT HEAVEN FEELS LIKE?
(there are even multiple forums with helpful folks to ask if you arent 100% sure)
You want them to ask random strangers on the internet for help? Assuming they realize that's okay and want to risk the potential abuse, where would they go? The OpenWrt forum doesn't exactly have "Help for Newbies!" stamped all over it. And don't forget that asking (and reading answers to) technical questions is a skill in itself. People write lengthy guides for how to do it.
step 3, flash it the same way you would a normal firmware update
What's a firmware update? Only tech-savvy users would think that a brand new product needs to be updated the moment you take it out of the box. And once it works, there's little reason to ever touch it again.
The OpenWrt installation guide has a warning at the top saying that installation could break your hardware. If the user realizes they want the OEM firmware option (unlikely), they will find the instructions unhelpful: "Open the WebUI of the OEM firmware with your web browser and install the OpenWrt factory firmware image file using the "Firmware Upgrade" option. Done.".
step 4 change the default password
Finally, something simple! Now the password is "123456" instead of "password". Security at last!
Sarcasm aside, I hope you can see that there really is a lot of technical knowledge and experience involved even in such a simple-sounding upgrade. To you and me, it's "grab the image file for your router's CPU and flash it in the usual way", but for people who don't know anything about routers it's a dozen-step process with plenty of opportunities for failure.