Schools don't purchase Chromebooks for powerful, long lasting electronics. They're cheap, minimalist devices that have all the capabilities that schools need.
It would be like lamenting over the end of Android updates of a $30 Android phone. The phone has already served its purpose being usable for a couple of months.
It works out $30/device/year. Google Apps for Education is $0/user/year. So I hardly think the 5 year EOL policy is going to destroy the value Chromebooks in education offer.
If anything is going to upset the balance it would be making Google Apps for Education not free. But Google has given no hint about doing so yet, and until they do...
Dell Inspiron 11 3000 @ approx $150/unit with no OS (Windows later). Which is pretty competitive to most 11" Chromebooks which come in at $150/unit with Chrome OS.
Obviously that "No OS" part is a gotcha, but School Districts are practically given Microsoft licenses for pennies on the dollar, so pushing an image of Windows 10 Enterprise isn't exactly a big ask.
The biggest downside of Windows isn't the cost of the devices, it is the staffing costs. With Chromebooks you may be able to spread one full time IT support tech between three or four schools, with Windows it might be one support tech for two schools, and they'll need to be shadowed by people handling systems like AD/Exchange/et al.
I will say that in my limited experience larger school districts do very well with a Windows solution, but smaller ones do better with Chromebooks. That's because larger ones can absorb the cost of the AD/Exchange/server admin easily, whereas for smaller ones you'll have to lose IT support tech staff to go handle that (which hurts end users).
Microsoft has been trying to combat some of these limitations with Azure/Office 365. They, again, give School Districts a huge discount and it allows the school district to outsource some of their staffing needs to Microsoft. Will it work? Time will tell.
I will add this: iPads have been an unmitigated disaster. Everyone rushed to buy them and then had no idea what to do with them once they got them. No money was spent on apps or lesson plans (that focused around an iPad), and even for generic "research" carts they're very underwhelming.
Both Chromebooks and Windows (w/Office) do what students and teachers need: Student email, student portal, research, typing, and spreadsheets.
Ever watch a student try to research something and type a report on that subject using an iPad? It is damn painful, many just start writing the information on paper, then writing up their report at the end, whereas a Chromebook and Windows machine (and MacOS in fairness) are full on multitasking.
Watch this video of someone multitasking on a Chromebook[0] and now an iPad[1]. The iPad is a toy by comparison. I won't even insult anyone's intelligence by linking to videos of Windows/MacOS, both are solid as a rock for multitasking too (MacOS just isn't "realistic" for budgetary reasons).
I don't think the main problem with Windows PCs is the cost of AD/Exchange/etc although it's a big problem. IMO the main problem is that even well managed Windows installs have a way of turning to shit (mine did recently). OTOH ChromeOS is the exact opposite: unless you put your Chromebook into developer mode you cannot muck with the OS at all. And even if you manage to somehow get your device into a bad state, reinstalling from a factory image is easy. So Windows might have more apps and be a more capable OS than ChromeOS but its support costs are too high for it to viable.
Windows doesn't have those problems if it is well managed. The second you get the wiff of a problem on a specific device, you just rebuild it from scratch over the network using WDS to select which installation you want, hit go, and walk away.
But as I said above, managing the services behind the scenes (inc. WDS and the images) costs money and smaller school districts struggle to absorb that overhead. I think all you've done is describe the same problem in a roundabout way.
At my brother's school the Chromebooks are universally hated. And his took a dive the other day even, even a factory reset couldn't fix what was screwed up on it so they replaced it.
The universe certainly has a great sense of irony.